Transmission/Differential Oil Change Basics

Transmission/Differential Oil Change Basics
By John Kuykendall

Learn the basics of changing the oil in your Miata’s manual transmission, repairing torn shifter boots, changing the shifter turret oil, and changing the oil in your differential. Do it yourself to save money and to make sure the job gets done right. Changing your transmission and differential oils regularly will help ensure long service life and improve performance and shifting action. Check your Miata Owners Manual’s for the recommended service interval for checking and changing transmission and differential oils. Or, change them when you are looking for an improvement in the shifting and performance of your Miata.

Transmission Oil Selection, Tools and Other Parts Needed

Transmission and Shifter Turret Oil – Mazda Specified Viscosity, SAE 75W90; above 50F, use SAE 80W90, API Service GL-4 or GL-5; high-performance substitutes, Redline Synthetic Manual Transmission Oil “MTL”, “MT-90” or equivalent. Some feel that MT-90 is closer to the Mazda-specified viscosity than MTL, others swear by MTL. Take your choice.

Differential Oil – Mazda Specified Viscosity, SAE 90 above 0F, API Service GL-4 or GL-5; high-performance substitutes, Redline Synthetic Gear Oil “75W90NS” (designed for clutch-type limited slip differentials) or “75W90” (for ‘94 and later Torsen LSD’s & non-LSD differentials), or equivalent.

Parts – Sealing washers for drain plug (trans and diff) and fill plug (diff only), Mazda OEM

Parts – Upper and Lower Shifter Boots, as needed, Mazda OEM. Check with Dealer for proper parts numbers for your particular Miata.

Supplies – Gasket sealant, 3M or equivalent; and, automotive pipe thread sealant.

Tools – Oil transfer pump, oil drain pan, cross-blade screwdriver, 14mm open-end wrench, 14mm crowsfoot wrench (optional), 10, 14, 23, and 24mm sockets, torque wrench.

Repairing Torn Shifter Boots and Changing the Shifter Turret Oil

Step 1: Place the transmission in Neutral and apply the parking brake. Remove the five self-tapping screws holding your console in place. There is one on each side of the console near the front corners, one under the ashtray, and two inside the armrest storage compartment.

Step 2: Unscrew and remove your shift lever knob. Lift the console cover up over the shifter and high enough so that you can see any wiring connectors underneath.
Step 3: Disconnect the electrical connector for the ashtray light (and for the power windows, if so equipped) by squeezing the ear(s) on the connector and pulling it apart. Carefully lift the console cover over the levers for gas and trunk releases, and place it aside.
Step 4: Pull the fiber insulation away to expose the large upper shifter boot and four fastening bolts. Place the insulation aside for reuse. Inspect the upper boot for tears, which are quite common in Miata more than a few years old. If torn, the boot should ideally be replaced as it isolates the passenger compartment from both heat and noise.
Step 5: Remove the four bolts fastening the upper boot with a 10mm socket, and move the arch-shaped metal bracket aside. You can see from this picture that my boot was ripped pretty badly. Since I already knew this, I had ordered a replacement. If your boot is OK, then just lift it up over the shift lever, to get it out of the way (you don’t have to remove the boot from the lever if you’re not replacing it.) Those replacing their upper boot can go ahead and remove it, which may be easiest if you cut the white nylon ring in half to release the boot from the lever.
Step 6: Here the large upper shift boot has been removed, exposing the shift lever and small lower shift boot, secured to the transmission shifter turret by three bolts. If you did not remove the upper boot in the previous step, the view will be similar except the upper boot will be pulled up over the shifter lever. Note that in this picture, you can see that my lower boot is also torn. This boot protects the shifter ball, mechanism and turret oil from contamination by road dirt and moisture, so it’s important that a ripped lower boot also be replaced. I just guessed that this one would be torn, so I ordered a new one of these too! Again, it is not unusual for the lower boot to be torn on Miatae more than a few years old. Note that in this image, you are looking through the hole in the transmission tunnel straight down at the ground! This is why replacing a torn upper boot is so important for isolating the passenger compartment from road noise.
Step 7: Make sure the transmission is in NEUTRAL. Remove the three bolts securing the lower boot and shifter to the shifter turret with a 10mm socket. Carefully lift the shifter up off the turret, being careful not to tear the lower boot if it is to be reused. In this view you can see the shifter pivot ball, the small nylon bushing at the bottom which engages into a recess in the turret, and the fatter upper shifter rod which is rubber vibration-isolated from the lower section. The two white connectors in the picture are the ashtray light (smaller) and power window switch (larger) connectors.
Step 8: Looking down into the shifter turret, you can see the rectangular boss with circular cutout into which the shifter engages, the shifter locating pin at the top, and the dirty, filthy old shifter turret oil. We’re here to remove that. There is a nylon bushing ring and washer inside the turret that you don’t need to remove. Note the mating surface with three threaded bolt holes – this is where you’ll apply gasket sealant when reassembling the shifter.
Step 9: This is an image of the shifter boots and shifter. The old torn lower shifter boot is at top left, having been removed from the shift lever, bottom. You’ll probably want to cut the torn boot in two before lifting the metal-backed mounting ring up over the tapered section of the shifter. It won’t go over the pivot ball. The new lower boot is shown at top center and the new upper boot is at top right. You’ll note that the shift lever tapers from the top into a larger diameter, then gets smaller again. This smaller center section is where the new lower boot needs to go. These parts are available from your dealer for about $20 each, or get them via mail order from Roebuck Mazda for about $15 each
Step 10: You’ll need to remove the old, contaminated oil from the shifter turret. You can use an oil transfer pump as in this shot, or a turkey baster or similar vacuuming device. Whatever you use needs to have a long, relatively slender tip or hose to get down into the recesses of the turret and reach all the old oil. Pump as much of the old oil out as possible, and dispose of it responsibly. Now, refill the turret with the proper quantity (80-95cc according to Mazda) of the same oil you use to fill the transmission proper. I’m using Redline MT-90 in my transmission, others swear by Redline MTL. Just make sure you use an oil with the specified lubricating viscosity (SAE 75W90 or 80W90). In my case, filling with 95cc brought the oil up to a level equal to the top of the rectangular boss within the turret.
Step 11: If you are replacing your lower boot, spread some transmission oil on the inside of the boot and on the outside of the tapered section of the shifter. Carefully stretch and slip the boot over the shifter, until it drops into the smaller diameter center section. Apply some gasket sealant to the boss at the top of shifter turret (whether you’re using a new boot, or reusing your old one). Clean the shifter pivot ball and end bushing, and lubricate with some new fresh transmission oil. Drop the shifter into the turret, making sure that the pin of the transmission engages the slot in the shifter ball. Don’t force it, as it should go in without too much effort. Secure the shift lever and lower boot with the three bolts removed earlier. Check for proper shifting action.
Step 12: If you are installing a new upper boot, apply some oil to the tapered shifter shaft again, and to the inside of the upper shift boot. Slide the upper boot onto the shifter and down as far as it will go, being careful to keep it in the proper orientation and to avoid stressing or tearing the boot. If you are reusing the old upper boot, you can now pull it back down into position. Fasten the boot onto the transmission tunnel with the four 10mm bolts, making sure to include the arch-shaped metal bracket under the rear two bolts.
Step 13: Position the console cover over the shifter, and reconnect any electrical connectors. Slide the console cover down into position, and fasten in place with the five screws removed earlier. Screw the shifter knob back onto the shifter, and you’re done. Check to make sure everything works properly.
Changing Manual Transmission and Differential Oil

Differential Drain and Refill
Step 1: Jack your Miata up high enough to be able to work under and place it on four jack-stands so that it is as level as possible. This is important in that it will help insure that you are able to completely fill the transmission and differential with oil.

Step 2: Locate the differential, which is a finned aluminum housing with the axles protruding from either side, just forward of the muffler, and within the rear suspension subframe. See the pictures if you are unfamiliar with its location. Note the yellow-painted filler plug head. Yours may look different, as this one is on a ’95 with Torsen limited slip.
Step 3: Identify the filler plug (yellow) and drain plug (lower) locations on the differential. The filler plug requires a 23mm socket or wrench, and the drain plug requires a 24mm. Unfortunately, these sizes are not in the typical home mechanic’s socket or wrench set. Don’t attempt the job if you don’t have the proper tools, go out and buy them now and you’ll have them for next time. Now for one of those “mechanic’s secrets”. BEFORE you remove the drain (bottom) plug on either the differential or transmission, make sure you can remove the fill plug! Just common sense, but you don’t want to remove the plug and drain your trans or diff only to find out you can’t remove the fill plug to refill it! Go ahead and remove the fill plug, and be prepared to catch any oil that might drip out of the fill hole due to the diff being overfilled or your Miata not being level.

Step 4: Now remove the bottom drain plug, and be ready to catch the draining oil in a catch pan or similar. If you have recently driven the car, the oil may be a little warm. The oil will probably also be quite dirty, particularly if the oil has never been replaced or if it’s been a while since the last change. Let the oil drain until it is just barely dripping out.
Step 5: Shown here are the drain plug (on the left) and fill plug (on the right). You may be able to see that the drain plug has some fine metal particles stuck on the end. The drain plug is magnetic, and it attracts the fine metal particles that result from the meshing of the metal teeth within the differential. It is quite normal for there to be small particles or filings present.

If there are large pieces or fragments of metal either stuck to the drain plug or present in your oil catch pan, then there MAY be some type of problem with your differential. Clean the plug of filings/particles, so that next time you change your oil you can tell if there has been unusual wear by the amount of particles present.

Note the aluminum sealing washers on each of the plugs. According to Mazda, these should be replaced when changing the diff and trans oils. If you didn’t have the foresight to purchase new ones, then reusing your old washers should be OK. Reinstall the diff oil drain plug and torque to specifications (29-39 ft-lbs.)

Step 6: Use an oil transfer pump or similar to pump one quart of differential oil into the fill hole of the differential. If your Miata is level, or preferably slightly “nose down”, you should be able to get the entire quart of new oil into the differential. The correct differential oil fill level is just up to the bottom of the fill plug hole. If the car is not level, oil will drain back out before you’ve pumped in the entire quart. Upon filling the differential properly, reinstall the fill plug and torque to specifications (29-39 ft-lbs.)
Transmission Drain and Refill

Step 1: Locate the transmission under your Miata, it’s directly behind the engine. There’s not much room under there, so make sure you’ve got your jack stands as high as practical. Locate the fill plug on the driver’s (left) side of the transmission, and remove. It’s a square-headed plug, which you can remove with a 14mm open-end, crowsfoot wrench or adjustable wrench. Note that it has pipe threads, so there is no sealing washer required. As before, make sure you can remove the fill plug before you remove the drain plug, and watch out for any dripping trans oil from the fill plug, in case it was previously overfilled.

Step 2: Locate the drain plug, which is a hex-headed plug located on the bottom of the trans, somewhat farther back than the fill plug. Remove it with a 24mm socket or wrench, and retain the sealing washer for reuse, if you didn’t purchase a new one. Be ready to catch a relatively large quantity of trans oil which flows out fairly quickly in your catch pan.
Step 3: Here’s a picture of the transmission fill plug (left) and drain plug (right). Again, the transmission drain plug is magnetic, so look for evidence of large metal particles or pieces on the plug or in the oil that is drained. Clean the particles off the drain plug before replacing.
Step 4: Once the transmission oil has fully drained, replace the drain plug with a new or reused sealing washer, and torque the plug to specifications (29-43 ft-lbs.). Fill the transmission with two quarts of new transmission oil, using an oil transfer pump or similar. Check that the oil level is just up to the bottom of the oil filler hole. Apply some pipe sealant to the threads, and replace the oil filler plug and tighten to the specifications (19-28 ft-lbs.) Note that since this is a square-head plug, you will need to use a 14mm crowsfoot wrench in combination with your torque wrench, as a standard socket will not fit. Your torque reading will be slightly inaccurate due to the longer arm of the wrench+crowsfoot combination, so keep it on the low side of the spec. To be honest, since I didn’t have a crowsfoot wrench I just tightened the fill plug until it was snug!
Step 5: You’re done. Check for any obvious oil leaks. Clean up, put your tools away and go out for a top-down test drive! For the next couple of days, check the driveway or garage floor under your Miata to make sure you don’t have any leaks. Check the oil level of the transmission and differential at least as frequently as specified in your Miata Owners Manual and top-up as necessary.

I previously had changed my fluids to Redline synthetics when I first bought my car used, filling the transmission with Redline MTL. Shifting action improved, particularly in cold weather. Lately, I had found that my shifting had been getting notchier and had been balking on the 1-2 shift on occasion at high RPM. This prompted me to change the transmission oil again after putting about 20K miles on the car.

After changing the turret oil, which had never been done since I owned the car, and changing the transmission to Redline MT-90, I find that the shifting is slicker, less notchy, but not really any easier than with the MTL. I expect that the MT-90, being heavier, may not make the transmission as easy to shift at lower temperatures as the MTL. Either Redline product will probably work. There is also no evidence that the standard transmission and gear oils won’t protect as well, but it appears that low temperature shifting action is noticably better (easier) with synthetics. Since you change them quite infrequently (maybe every 25,000 to 50,000 miles), the added cost of synthetics is probably justified by the benefits.