I am helping a buddy change the fluid on his 1986 F150 (5.0, 4x4, with four-speed manual tranny). He has a Chilton manual that indicates 85W-90 gear oil for the tranny, but when I called Royal Purple this afternoon to see if they had a substitution the technician told me that his source indicated 80W gear oil and that I could substitute synthetic 5W-30 motor oil since it has similar viscosity. Apparently the weighting scale for motor oil and gear oil are different...who says that you don't learn something new everyday.... Anyway, before I go with it and risk screwing up his tranny I figured that I would see what the truck community has to say. Does this sound like it will work? I looked up a comparative viscosity scale for SAE and gear oil and the RP tech's info checks out, but it just feels weird to be putting motor oil in a tranny.

Any help would be greatly appreciated.

Also, is the manual 3-speed plus o/d an M4OD or an SROD? I haven't really been able to figure that out and the numbers stamped on the casing are worn to the point that I cannot clearly make them out.

Thanks again.

 

Could be the TOD 4 speed, it takes gear oil. Do you want to believe the manufacturer or some guy? This is the TOD in the foreground:

 

Thanks for the pic Sean. From the appearance, I don't think that its the TOD. I don't know if it helps, but it looked like there was only one drain/fill hole, unlike the two that I was expecting.

Also, I'm assuming with the single port that I would just suck out the old lube with a suction gun?

I'll stick with gear oil. I'm assuming that I could use a 75W-90 in place of the 85W-90 since the viscosity at operating temperature would be the same. Sound reasonable?

Thanks again