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Question about using Synthetic gear oil and weights

rdv350

Sr. Member
Joined
Aug 22, 2007
Messages
429
Loc.
Scranton, PA
I am in the process of changing the gear oil in my diffs and the trans and transfer case. From what I have been reading in past threads almost everyone recommends using a lower weight oil like 75w-90 for colder temps. Everyone says that a heavier oil like a 75w-140 will get too thick when the temps drop below freezing. Well I was looking in the maintenance manual for my F-350 yesterday and they recommended 75w-140 synthetic gear oil of the rear, so what gives? Why is this recommended for my truck, but I can't use it in my bronco?

My other question is, no matter which oil weight I go with, is it ok to use synthetic gear oil in the trans, t-case, and diffs?
 

Apogee

Contributor
Bronco Guru
Joined
Nov 26, 2005
Messages
6,030
Dual weight oils like 75w-140 have the advantage in that they have viscosity characteristics of the lighter weight at lower temperatures and the higher weight at elevated temps, loads, etc. You should be fine running a 75W-140, although I'm sure if anybody has had issues with their rigs they'll chime in.

As for whether it is okay to use synthetic oil in your rig, I can't see why it wouldn't be? Synthetic oil is still a chain of hydrocarbons, just like regular oil, albeit a more heavily processed version of it. If I'm not mistaken, synthetics usually start as a gas like propane from higher up the distillation column so that it is pure and doesn't contain any sediments/contaminants and then it is modified to create whatever synthetic they're looking to produce. Synthetics are still petroleum products made from dead dinos.

It's expensive and some will say it's worth it, others will say it's not. That's something you'll need to decide for yourself.

Tobin
 

Redneck

Jr. Member
Joined
Oct 24, 2005
Messages
114
Loc.
Jefferson City, MO
Back when I was working in the automotive field (30 years ago) and synthetics were new we were told by the vendors not to switch from standard oils to synthetics unless it was a new vehicle or newly overhauled with new seals. The vendors claim that the synthetics were so much better in lubricating that old seals would start leaking. This was especially emphasized for the engine, I don't know how axle seals and such would react.
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,821
For decades the standard gear oil was 90 weight.
Then they started multiweight and 75-90 was common.
Now the OEM are specing 75W140 in nearly every application. I have noticed this change over the past 5 years or so.The 75 part gives flowability when it's cold out but the 140 takes the heat better. Just about every truck out there now specs out 75W140 synthetic as the rear axle lube. 5 years ago about the only thing that ran that was the half ton chevy truck as a bandaid so they could use a rear end that was too small for towing.
 

Apogee

Contributor
Bronco Guru
Joined
Nov 26, 2005
Messages
6,030
Broncobowsher said:
5 years ago about the only thing that ran that was the half ton chevy truck as a bandaid so they could use a rear end that was too small for towing.

Duct tape didn't work, so they had to try something else :cool:
 
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