I have a 86 f150 its the base model so im pretty sure it takes 80w90 and tomorrow i wanted to change out the oil fluid and i wanted to ask someone with experience to see if its cool if i use royal purple 75w90 max gear lube?

 

75/90 is synthetic, all the newer trucks are using it, you never have to change it. On heavy trucks they use it and they recommend 500 to 750 thousand mile oil changes.

 

I have an 03 ranger and fords says the diffs.use 80w90,but when I change the fluid I'm putting 75w90 val.syn power the ford shops say no big deal between the two and beside the 75 is sposed to be better for your mpg.

 

I'd go for 75w140 instead for the rear end.

 

75/140 is to heavy for high speed gears and cold weather, stay with 75/90.

 

75W140 is now factory fill in all Ford/Sterling axles. The "W" rating of 75 means its fine for cold weather.

 

Ford recommends 75/140 in they're axles now? That's interesting, is it stated in the manuals? Trust me, 75/140 pumps harder than 75/90 in the cold, I use both.

 

Yes, it's in the owners manuals. 75W/140 is in theory, better in the cold than the old 80W/90, but it of course depends upon how cold is cold. SAE viscosity tests are at a particular temperature, -40C for 75W, -26C for 80W. According to Royal Purple's data sheet, 75W/140 has a lower pour point than 75W/90 (-69F vs -60F). It is possible that 75W/140 will have a slightly higher viscosity at 0C (32F). You can do the VI calculation on any oil from the manufacturers data sheet.

 

Well there you go, Ford went to lighter oils in their gas engines to save fuel and heavier oils in their diffs. Must be because they were breaking the fluid film when the oil was hot with their bigger engines.

 

I think what happened is that tow ratings went up along with engine torque ratings, so the axles need better protection at the extreme end. F-150's (without the non-SuperDuty 3/4 ton "payload package") still use an 8.8 inch ring gear, yet are rated to tow twice the trailer weight compared to 20 years ago.

 

75w-90 and 75w-140 are both 75w at cold temps. The 75w-140 will protect better at high temps since it won't thin out as much.

 

The difference between 75W-90 and 75W-140 gear oil is around the same difference between 10W-30 and 10W-40 motor oil at operating temps.