The grinding noises associated with hard shifting due to syncro wear may be reduced a little by changing the transmission oil. Many owners have found that Redline MT-90 transmission oil reduced syncro wear and aided shifting feel. Once you have changed your transmission oil, the effects will not be immediate; the new oil takes a few days to work itself into all of the bearings and you will notice the shift quality increase slowly.
Details on changing the transmission oil are available in many threads on MR2OC.com, such as this one. A brief summary is below.
- 1985-86 have C50 transmissions; 1987-89 have C52 transmissions which are not prone to Fifth Gear Pop Out. The two are virtually the same, except the later transaxles have humps for the starter on both the intake and exhaust sides.
- All supercharged models use the E51 transmission. It is very similar to the E153 used in the MR2 turbo and most FWD V6 Toyota transaxles.
- The drain plug is 15/16". A 24mm socket will work but is ever so slightly too large and can round off the bolt.
- The MKI NA transmission uses 75W-90 GL4 oil; it does not have yellow metal synchronizers.
- NA uses 2.7 US quarts.
- No limited slip differential was available as a standard option, however any LSD from any C-series transmission (MR2 Spyder, 2000+ Celica GT-S, Lotus Elise, etc.) will swap in.
- The average five-speed C-series transaxle weighs about 70lbs with shifter bellcranks and bearing/fork, but no clutch, fluid, or axle stubs.
- MR2s have intermediate axle stubs that spline to the differential and bolt to the CV tulip joint on each halfshaft. The tulip joints for some FWD Corollas with the same transaxle are integrated with the CV joint cage and spline, and do not have this intermediate bolted connection, however they may be interchangeable.
- AW11 MR2s made before 06/1985 use a 200mm clutch disk, while cars after 06/1985 use a 212mm disk. The clutches are interchangeable, however each clutch must be used with it's corresponding flywheel.
- Gear ratios (C50):
- Gear ratios (C52):
- The MKI SC transmission uses 75W-90 GL5 oil; it does not have yellow metal synchronizers.
- SC uses US 4.4 quarts (although not many get that much in there).
- No LSD was available standard with federal supercharged models, however any LSD for an MR2 turbo will swap in. Supercharged overseas Corollas also had a viscous LSD factory option that fits.
- The weight of an E51 transaxle is approximately 130lbs. E-series transaxles were Toyota's heavy-duty transaxle for decades and are very hard to break, however they are also extremely heavy.
- E-series transaxles use a 225mm clutch disk.
- Gear ratios:
Either Redline MT-90 (around $8 per quart, GL4 rated) or fully synthetic Penzoil Synchromesh (GLS rated (yep, really); around $5 per quart from AutoZone -- look around, as the staff usually don't know it exists). In general the MTL is better suited to colder climates, whereas the MT-90 works better for hotter ones. The MTL will sometimes have a slight amount of gear rattle in hot conditions.
Some excellent instructions are at MR2-Howto.
Alternately fill from the top. Remove the intercooler, and underneath is a 24mm bolt that goes straight to the transmission. There is a similar, smaller bolt on the C-series transaxles (next to the transmission vent). Make sure you out the correct amount in, remembering you probably didn't get it all out in the first place -- around 3.5 quarts should do it for the E51, 2.5 for the C52.