Would ther be any problems with running 75/140 in the diff's. I am thinking of using Amsoil 75/140 but book says to use 75/90 I think.

 

probably too thick for the cold enviroment you live in.

 

I thought there was something about the 75/140 with towing a lot? I would be curious to know as well since im about to pick up a PML rear diff cover and well obviously change my fluid..

 

I am running the 75w-140 in N. Illinois, and it has been below zero every night for 2 wks. I am also running it in the final drives and front axles on MFWD tractors as well.

I have a couple partial bottles on the shelf where there is no heat--and I was curious as to how thick it was in this weather. It shakes in the bottle, and pours well, so it is thin enough--Not much thicker than Delo 15w-40 was. I have been very pleased with the oil--The temp reduction in the diff while towing was unbelievable--with 75w-90 Valvoline syn in the diff it would be too hot to lay my hand on for more than a few seconds--with the Amsoil I can leave it on there all day, it is warm, but definitely not hot.

 

Is the Amsoil Synthetic?

 

I believe it is.

 

Yes, your right the fluid is synthetic. Just about every AMSOIL oil is synthetic.

I don't run the 75/140 but I do run the 75/90 and it has done me well so far.

 

I run Royal purple 75w90 in the front and 85w140 in the rear. Very good results over the last 5 years.

 

Amsoil has a 75W-110 Severe Duty synthetic now, would be a nice compromise for those that don't tow much but want better protection than a 75W-90.

 

I used RP 85w140 in Co. and it was great in spite of all the naysayers. My test reports were the best of all the lubes I tried. What wt. you use depends on how you use your truck. If I used the truck hard at all I'd use a minimum of 75w140 unless I lived at Minot or equally cold place. I just haven't seen what I'd call adequate protection from 75w90 in these trucks. Craig

 

Ive got redline 75-140 in both front and back axles, it gets to -35 here, and the diffs have not blown apart yet

75-90 and 75 140 have close to the same poor points in cold weather, from what ive read

 

I use RedLine 75W-140 in the 11.5" AAM

 

Well looks like ill run the 75-140 then.

Also im seeing everyone post either Amsoil, Redline, or Royal Purple what about Valvoline synthetic 75-140?

 

So my question here is for everyone running the 140 over the 90. What's the huge difference between the two, viscocity breakdown? Is their better protection in the 140 fluid?

 

Difference between a 140 weight and 90 weight gear oil is film strength and a thicker barrier of fluid so to speak.

You might lose a little MPG with a 140 gear oil, but it would be so minimal that it couldn't be calculated. The extra protection of a 75W-140 is more than worth it IMO.