Manual Tranny Oil
 

Ok so, I drive a Yaris. I changed my motor oil to Mobil 1 synthetic. Now I want to change the manual tranny oil to synthetic as well. Will this have an effect on mpg? Second, will using a higher octane gas (factory recommends 87 and I use that) have any effect? Thanks!

On some other cars I had, I had MPG gains using Redline MTL synthetic lubricant in manual transmissions on FWD cars.

Fuel, use the lowest necessary to get the job done. In come cases, pump gasolines with higher octanes will leave carbon deposits in the combustion chamber. Might not be in all applications, but it's just not of any benefit when it isn't necessary.

If the factory calls for 87, run 87. generally (but not universally) you will not gain anything by running premium.

My car is an example of the opposite: it calls for premium but will run fine on regular. however, it runs a little smoother, is more responsive, and get better mpg when running premium because the ECU can advance the ignition timing more. I did the math and it's actually more cost effective ($.01 per mile) to run premium.

I would ask Toyota first. With such a new car, which I would assume still has a warranty, it could void the drivetrain warranty if it somehow causes an issue with wear.

If that isn't an issue for you, give it a shot. Just get something that is a similar viscosity to the original fluid in the car. If you don't shifting when the car is cold can get very difficult.

Check your owners manual for what type of fluid is called for and run the thinnest synthetic you can find that meets the spec. Many manual trans these days use ATF.

Yeah you should have no problem running Mobile 1 Gear Lube in the tranny probably calls for an 80-90 weight like my xB. You will find that it will shift a little different at first so don't try to shift too fast or you may feel some gear grinding until the syncros adjust to the new oil. You probably will see an increase in mpg right after you change the gear oil. Seems the factory gear lube is not that slippery.

The owner's manual and the factory repair manual both specify GL4 or GL5 75W-90 oil and nothing more. So I think I am safe, yes?

yup just read the Mobile 1 can / bottle and don't over fill it! Make sure you open the filler plug before you drain the tranny because you can't put more oil in after you drain it if you can't open the plug.

Use the GL-4. I'm not *sure* about the Yaris, but in the xB the GL-5 is actually corrosive to the brass synchros. Plus the GL-4 is a slightly lighter weight.

Mobile 1 Smobile 1 just get a couple of quarts of Synlube!