1997 Ranger Transfer Case fluid

My son has a 1997 Ford Ranger. It has a newer transfer case on it. On the case the installer wrote to use 10-40W motor oil. The fluid level port has use ATF fluid stamped on it. Is there any situation where motor oil is needed for this fluid application? The transfer case is a Borg Warner 1354 Mechanical.

The transfer case fluid should be Dexron/Mercon ATF.

But since transmission fluid is just 10 weight oil with the proper additives, using a 10W30 oil still provides the proper viscosity when cold but provides a thicker viscosity when it heats up. Just like it does when it’s used in the engine. And since the transfer case is a mechanical component with no hydraulics or clutches involved that require ATF, I can’t see a problem.


Thanks Tester. I just can’t figure out why the installer would spend the effort to write this information with a Sharpie on the case. It has oil in it (small leak by the rear drive shaft) could this (using oil vs ATF) be suggested to mask a bad seal?

Only if it is a ‘high mileage’ oil with seal conditioners in it. Otherwise, no. Check the fluid in the normal fashion, and see if it is red or brown. Red transmission fluid has a distinct smell as well. Should be easy to determine if the fluid is ATF or motor oil. Replace with the matching.

You can’t mask a seal leak with switching to motor oil. Both the ATF and the 10W40 motor oil are at the same viscosity when cold. So they’ll leak at the same rate. And since the transfer case is a vented component, no pressure is built in the tranfer case. So using oil wouldn’t make a difference.

A seal leak is a seal leak.


Thanks again. I guess I’ll just stay with the oil.

As my original question was not fully answered … why would the installer specify 10W40 oil … I tracked this further. I contacted RSGears who sell this particular transfer case their response was as follows:
Good morning,

The Borg Warner 1354 transfer case you are working on is spec’d to use ATF as the lube fill. You should not use 10W40 motor oil. There is an internal pump driven off the mainshaft that supplies lube to the internal parts. Motor oil is too heavy a viscosity for this pump to work properly. You have 2 problems here and they are not linked. The rear oil seal leak has nothing to do with the motor oil, which being heavier viscosity than ATF will actually leak less. The oil seal problem is the result of either damage to the seal when the drive shaft was installed, or problems with the drive shaft balance or worn or bad U joints. Use only ATF in this unit and change the rear seal.

Best regards,

The only plausible reason I can see for the mechanic to specify 10W40 was to mask the rear seal leak when he installed the rear drive shaft since the coiled hydrocarbons in the multi-weight oil, uncoil and retain viscosity at higher temperatures a property not shared with ATF. The only other reason I can deduce is that the mechanic didn’t have any ATF around, didn’t feel like going and getting any so just put motor oil in … but I doubt that he would advertise this by writing it on the transfer case with a sharpie.

Then you should contact the installer with the information you have and ask why do they recommend using 10W40 oil instead of ATF. They wrote it on the transfer case for a reason.


If I knew who it was I would have started at that point, believe I’ll just go with the ATF and fix the seal.