OT - A question for the oil guru's on this board

Hello All,

From time to time I have an OT question and in the past you have been very helpful, I have another question for you all.

I recently purchased a 2008 GMC Envoy with 160,000 miles to tow my 3,000 boat and run errands in (it's not my daily driver). We call it our "farm truck".

I decided to change fluids, including the front and rear differential. GM spec's 75W90 for both the front and rear diff.

I happened to have some Valvoline fully synthetic 75W140 laying around so I used that instead for both front and rear. Some of my friends have warned me that I am inviting disaster and that I need to immediately drain the 75W140 and refill with 75W90.

I didn't think it would be a big deal, but now they have put doubt in my mind.

I know that some of you are well versed in oils, can you enlighten me about this ?

Thanks !!
 
(quoted from post at 06:31:29 09/09/21) Did they specify their details of concern?

Yes, most were silly.

One guy is convinced that I'm going to "blow the seals out" of my diff's. I don't pay attention to that one.

Others say that GM spec'd 75W90 for a reason.

But one is concerned that the thicker oil will not carry away heat, and will not flow into the bearings.
 
The oil will retain it's viscosity in the range of 75-140 throughout the temperature range. In normal use
there shouldn't be a problem. This oil should get better results at high temps than lighter oils at the same
stress levels. Everyone has an opinion concerning oils.
 
The specific heats for 75W90 and 75W140 are different by about .006 BTU/lbF for the range of temps this will be exposed to. This will be insignificant for cast iron and steel in ambient conditions.
 

Some makes use 75/140 as a upgrade when towing heavy loads I see no reason why it would not work when replacing 75/90.

Dodge use to state in the owners manual when used for towing drain and put 75/140 in the rear differential. I have drained the diff on them off the show room floor and put 75/140 in them.

Any truck I own gets 75/140 as they all tow my tractors.
 
Ok, seems like I'm good to go.

Thanks for all the replies.

I'm considering the diffs done and moving on to other projects.

Thanks !!
 
I own and service our 2008 Envoy Denali. If I remember correctly the owners manual states to use synthetic gear oil in the differential. If I remember I'll take a look but I'm sure that's what I used when I did ours.
Checked my manual 75w-90 synthetic is what it calls for.

This post was edited by Eman85 on 09/10/2021 at 08:36 am.
 
Syn oil is higher detergent and can wash out seals is one.
The other is the possible effect of any reaction in a limited slip diff.
 

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