I have made the decision to put Mobil 1 gear oil in my front and rear differentials on my 89 F150. I believe Ford recomends 80-90 gear oil for these. Should I use the 75-90 Mobil 1 or the 75-140 Mobil 1? Am interested in flowability as the front end only gets engaged in the winter. Dino 80-90 is super stiff in those -20 F mornings and takes awhile to get things loosened up. Am thinking the Mobil 1 will be better in this regards.

Thanks

 

I think the flowability of the M1 offsets the 5W difference, especially in the front end you only use in the winter. I don't suspect it would be a problem in the rear end either, but take my opinion for what its worth - I'm not a mechanic or specialist on lubrication.

 

After reading the original post again I had the following thoughts. The 75 140 will thin less at operating temperature than the 75 90. So it's a tough call. I would think the 75 140 would work better for protection, but maybe the gears need the oil to thin to work at its best. Tough call.