Looking at 75w90's

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Going to work on doing my 100k service on the F150 this summer and one of the jobs is the front dif. I'm sure its in just as bad of need as the transfer case was. Spec is 80w90, I think I'll go with the 75w90 synthetic alternative but I'm a little torn on which one to get. Motorcraft and Amsoil run about the same price but I can save some money going with SynPower or M1 which are also about the same price as eachother and a couple bucks less per quart than Amsoil.

In the past I've had no troubles with Synpower. I did have some wheel bearing issues on my Mustang after switching to M1 which I cannot rightly explain as levels were fine and mileage wasn't too excessive at that time. Is there any real difference between the two? I have to imagine both are using grp III+ these days, and both seem highly reputed. Truck seems most 4WD action in the cold of winter so I do think the 75w will be a slight advantage.


Addendum: Looks like M1 is very slightly less impressive on paper.
SynPower:
VI: 150
PP: -48C
Vis 100C: 15.6
Vis 40C: 100

M1:
VI: 146
PP: -39C
Vis 100C: 14.6
Vis 40C: 103

I know numbers are FAR from the whole story but it seems the SynPower is marginally superior?
 
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I went with Mobil 1. Pep Boys has coupons from time to time offering 25% off and I loaded up on it then. Both my Jeeps have it in them, and I've used it in several other vehicles over the years. I never had a problem with it.
 
I've been running Supertech Synthetic 75w90 in the front diff of my 08 Wrangler, and 85w140 in the rear. It seems to be serving well with no issues. I do some towing with that Jeep, too.
 
Originally Posted By: CleverUserName
Delvac synthetic 75w90 is GL5 and affordable in gallon jugs.


It’s now Delvac 1 … have had that in G80 for 2-1/2 years now …
 
Originally Posted By: Patrick0525
@4WD Delvac doesn’t have friction modifier. Did you add it to the gear oil? And how much?


I’m also running D1 75w90 in a G80. This type of LS diff. doesnt require any friction modifiers.
 
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