That's interesting - I must have completely missed Katman's post. That is odd, because everything I read yesterday says the opposite. Engine oils need to be stronger than gear oils. You can use engine oil in transmissions, but not gear oils in engines.
The only difference I've seen in the MTF is the supposed additives to lubricate the synchros. Gear oils seem to be too thick for the synchros, but thinner oils should do the job just fine.
You have to remember that most of have light weight toy cars (let the flames begin) and motor oil may be fine in the tranny. But just think about it for a minute, why do you need all the additive that an engine needs in your tranny? You don't! In the tranny and differential and transfer case, you need heavy shear load and sliding protection. In an engine, you have have an oil film between most of the parts.
Kevin A Thornton
For Speed Equipment, Nitrous Express firstname.lastname@example.org
the main reason for useing motor oil in your tranny is that motor oil is pretty mild when it comes to reacting with yellow metals, I don't think any car manufactur recomends useing motor oil in the tranny in any new cars, it shouldn't hurt them to use it, but MTF that is labled as safe for yellow metals, or recomended for Honda's is most likely what you want, but motor oil will not last as long, they recomend chainging motor oil in your tranny every two years/60,000 miles, gear oil trannys can go much much longer, like on VW's they recomend changing it when it's contaminated, or tranny is worked on.
How sure are you about this stuff? It seems to me that if they both start from a similar base and then engine oil get's additive's to enhance some of it's protective qualities, that the sheer qualities and those types of factors would be present in the oil, irrespective of the other additives. It makes me wonder if it isn't more marketing issues than real physical issues.
What katman said about "toy" - ahem - cars may be correct in a way. Fluid type may be more dependant on the type or use of vehicle than on anything else.
I know that a lot of manufacturers do recommend engine oil as their MTF. I had an automatic Toyota Corolla and the owner's manual stated that the manual shift version required 5W-30 motor oil. Obviously this was a light-duty vehicle.
High HP cars, trucks, high-torque diesels, and big rigs are most likely going to need fluids that are a little more heavy-duty in order to last longer and to protect parts.
Well here's what I am puzzled or curious about. I am running engine oil in the transmission on my Honda's. However I've always used gear oil in differentials and transmission's, as that was what was recomended. It seemed to me like when I put the engine oil into the Honda, it has a lot less viscosity than any gear lube I've ever used. I figured it is a partial part of the reason the Honda is able to do better on mileage. However, I typically don't change gear lube unless I have to and I've never had a problem with it. Now I'm wondering if I should try to find an equivalent weight gear lube, instead of the engine oil, so I don't have to change it, or if I should just change it and quit whining, or if I don't really need to worry about it, either way.
What got the balling rolling in that thread was this:
3...I probably should change my transmission fluid. The owner's manual calls for gear oil, but the viscosity level of gear oil is like 75w-90. This seems really "sticky" and I especially noticed it during the winter. In -20?C/-4?F the shifter felt like it was moving through glue. Is there lower viscosity gear oil I can safely use? What are some options?
MetroMPG pointed out that the Metro's manual is actually wrong and that GM Synchromesh should be used - much much thinner than gear oil going by his tests. He had tried the gear oil in his first Firefly and actually ended up crunching synchros because it was so heavy.
I also drained the FACTORY fresh gear oil from the Blackfly when I got it and compared it to synthetic 75w-90 by timing how long it took to pour a measured amount of each through a filter.
That's how I figured out that what came in the transmission from the factory was definitely NOT 75w-90. What came OEM was much, much thinner and comparible to 5w-30 engine oil, based solely on its pour rate.
The semi-synth GM synchromesh tranny fluid I ended up using had a pour rate close to 5w-30, based on my garage floor science test.