Gear Lube Guru's,

-Curious to know the ill effects of using ATF in a manual tranny, over a thinner MT fluid like MT90.

-Why would this be recommend to someone?

Reason why: I was having problems getting my car in and out of gear in cold weather for a week or so after getting my manual tranny replaced. The mechanic drained the oil and put in a thinner lube and I've had no problems since. While I was there, one of the techs w/ a mustang mentioned that it is recommended that he use ATF in his manual tranny. Lead to the 2 questions above.

 

Some manufacturer's specify ATF in manual tranny's because it is thinner than gear lube. The ATF will flow better in colder weather (why you're shifts are better) and will give you slightly better fuel economy.

In any manual transmission and especially the ones that don't recommend ATF, you will have a higher wear rate on the internal components when temperatures and loads are high as ATF doesn't have the necessary chemistry to protect well in these situations.

Prime example: BMW specs ATF in their 5 speeds throughout the 90's. For normal street driving, this works out. If you drive agressively or are a track junkie, the general consensus is you're nuts if you don't run a gear oil.