Motor oil in auto trans? 4

[Metalguy]

Does anyone have any info on the use of motor oil (especially synthetics) in an auto trans, such as a GM 4T60E? Most motorcycles use a wet clutch similar to those in a car automatic, and the engine oil is also the clutch oil. Motorcycle oils which do not have "friction modifiers" are frequently specified.

I have called several oil companies and racing trans. (autos) shops, and all of them would not recommend it. But none of them could/would say why with any certainty. They all agreed that seal leakage should not be a concern--their "suspicion" was possible problems in the clutch area.

 

[TheBlacksmith]

Motor Oil - cleans, cools, lubricates, seals (piston rings)

ATF - usually has friction modifiers that enhance clutch performance. (Some modifiers increase friction for firm shifts like type F, some decrease friction for smooth shifts like Dexron.) I suspect that the shops are right, motor oil will cause slippage or harsh shifting and damage.

Blacksmith

 

[drwebb]

In no particular order:
1) The viscosity would be wrong. Dexron specifies a narrow viscosity of at least 6.8 cSt @ 100C. This would be motor oil SAE viscosity of about 25 and is not readily available in a crankcase formulations.

2) The viscosity index would be wrong. Generally crankcase formulations are optimized for high T viscosity control, whereas ATF formulations are optimized more for low temperature control. For example, Dexron calls for a maximum viscosity of 20 KcP at -40 C, while a 0W SAE grade oil allows 60 KcP at -40 C -much too thick.

3) You would have much more dispersant-detergent than optimal/needed. Crankcase oils are additized to maintain lubrication in the presence of combustion contaminants, which are not present in ATF. The high levels of calcium-based detergents used for crankcase oils are known to promote glazing of the paper/ceramic friction materials of ATF clutch plates.

4) The additives would be wrong. The antiwear additives for crankcase oils are optimized for valve trains and are typically Zinc based, which has much the same pore-clogging properties as calcium. ATF antiwear additives are usually more 'geared' toward extreme pressure load carrying.

5) The friction properties would likely be very wrong. The main design parameter of conventional ATFs is to maintain a both moderate and stable friction level (coefficient of friction) at the paper/steel interface for the longest possible time. Motor oils by contrast are designed to _minimize_ friction at the steel/steel interface, which is also a totally different microenvironment.

The bottom line is that the chemistry of these two lubricants makes them different- albeit related- animals. I am not familiar with the most modern MC clutch designs, but traditionally they have used rubber-based friction materials that are much different than Dexron-requiring friction materials of almost exclusively paper/ceramic composites. Maybe MCs get away with motor oil because being manually actuated they can control slippage by rider input to adjust for changing lubricant properties, thereby being less lubricant-sensitive- but I'm speculating. What I do know is that before the advent of electronic transmission controls it was difficult to adjust clutch slippage on the fly, and so the fluid and transmission were designed as a package to maintain stability together.

Your query begs another- why whould one want to? Unless one had inside sources there would seem to be no economic advantage of substituting synthetic motor oil for ATF. If it is the allure of the "synthetic" designation, then there are "synthetic" ATFs available in the marketplace. But don't think I am trying to sell you on not trying it, I am just answering the question of why it is expected to give disappointing performance.