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Old 03-23-2010, 08:52 AM   #1
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Synthetic gear oil question

I have an 04 Chevy Silverado.
If I change the rear differential oil will it run cooler using
full synthetic?

Has anyone here changed to synthetic gear oil?
Any links to testimonials or test results?

Thanks!
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Old 03-23-2010, 09:51 AM   #2
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Check your manual. My 01 Ford F150 came with synthetic in the rear diff, and the manual states not to do anything ever to it. I would have thought Chevy would have caught on, but I don't know. As for running cooler, have no idea. In my thinking, no. There is really no way to cool the diff fluid. I guess if it cut down on friction, it wouldn't generate as much heat. Just my 2 cents.
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Old 03-23-2010, 10:04 AM   #3
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We tried running synthetic in our Jeep differentials as they really see a lot of abuse but for that reason we ended up having to change it a few times a year too so it didn't work out to be worth the money. I run Lucas gear oil in all our differentials and in the final drive of the outboard motor on the boat. In the case of our Jeeps and the last motorhome I did notice less noise with the Lucas oil but I don't believe it's synthetic. My mind isn't convinced that the synthetic is better for the money spent.
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Old 03-23-2010, 10:13 AM   #4
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What got me thinking about this is some months back a poster here
said the "pumpkin" had gone out in his Chevy Suburban.
I don't remember if he said anything about synthetic or not.

The only reason I would change it would be if it was supposed to
reduce friction and heat.
Also synthetic does not thin out as much when it is hot.

I'll read the manual and see what they recommend.
I was just wondering......

After my last trip a couple days ago-
100 miles on interstate at 65 MPH and air temp of mid 60s.
When we got home after about 10 minutes I thought about
this and crawled under the truck.
The diff was hot, like a car hood on a hot sunny day.
I could leave my hand on the rear cover but it was mildly
uncomfortable.

I guess this is normal.

In June we plan a big trip to the south west.
We'll have days of many hours of Interstate at temps in
the 80s.
Just wondering if a gear oil change would make any difference.

I know asking a question like this is like asking
which tires are best or
which engine oil is best.
EVERYBODY has an opinion!!
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Old 03-23-2010, 10:34 AM   #5
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Yeah, the diffs being hot is normal. For rigs that see lots of harsh conditions, like our Jeeps or tow vehicles doing a lot of pulling I think the best advice is what mechanics will always tell you. Keep the fluids changed and clean. Some diffs are easier to change the fluid on than others. Ford's 8.8 axles (of which I have under the rear of my Jeep) has a convenient fill hole up high in the front. Many of the Dana axles have a single fill port put you either have to loosen the cover bolts to drain them or pump the fluid out through the fill hole. I have replaced the covers on my Jeep with heavy duty covers that have drain and fill plugs built in, makes it really easy to change the fluid. If it were me and I was towing a lot I would change my diff fluid at least every other year for piece of mind. It's not all that expensive and the oil you choose to run as long as it meets specs is probably not all that important.
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Old 03-23-2010, 10:50 AM   #6
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For the small difference in cost as you only need about 2 liters, I'd recommend going with the synthetic. No one will argue that's it's the best for reducing friction. That's what I use in the diff. of my drag car.

If you're adventurous, you could drill and tap (1/2" NPT) in a low point on the differential to make future changes a lot easier.

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Old 03-23-2010, 12:22 PM   #7
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all the newer GM's use synthetic from the factory...pretty expensive- $30 per bottle...rear diff takes 2 1/2 bottles
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Old 03-23-2010, 12:30 PM   #8
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Some people have reported better fuel mileage; especially in winter when lubricants are thick. There are also "watchouts". If you have a locking rear differential, my understanding is that they require a special lubricant with "friction modifiers". The F150-Online forum mentions this. Frankly, I wouldn't pursue synthetic in differentials and transfer cases unless you live in a cold climate.

Similarly, I used to be a user of synthetics in my engines for years. They do have better cold start advantages IMO; but I'm talking 0 degrees or colder. Recently, I've gone back to Ford Motorcraft oils and filters and they work just fine. I go 5,000 between changes and it never moves off of the full mark. The Oil Guy tests (google him) show that Motorcraft holds up extremely well and is a lot more cost effective.

Not knocking synthetics, I think they are superior. The question comes down to whether you really need it and the cost effectiveness. If it gives you extra peace-of-mind, then it may be worth it.
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Old 03-23-2010, 01:41 PM   #9
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Quote:
Originally Posted by acadianbob View Post
There are also "watchouts". If you have a locking rear differential, my understanding is that they require a special lubricant with "friction modifiers".
More accurately friction modifiers are required by some manufacturers if you have a limited slip differential, not lockers. LSD work on an internal clutch system in the diff that senses the loss of traction and locks up and they do require friction modifiers. True lockers (Toyota TRD, newer Jeeps with air or electric lockers, Eaton E-lockers, ARB air lockers) do not use friction modifiers as they have no clutches. When a button is pushed they lock the axle solid like a spool.

Your owners manual will state if you need a friction modifier. You add the modifier to regular gear oil and it usually comes in a smaller bottle or tube.
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Old 03-23-2010, 02:07 PM   #10
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"What got me thinking about this is some months back a poster here
said the "pumpkin" had gone out in his Chevy Suburban.
I don't remember if he said anything about synthetic or not."


That was me ;-(

Synthetic came OEM in that diff from Chevy...and it burned up good. Following the warranty repair service, I actually drained the new replacement oil and went to AMSOIL Synth Gear Oil after doing a bunch of research. I love it ! I learned a lot from that whole ordeal.

I added a Mag-Hytec diff cover (holds more oil, has magnetic dipstick, and a drain hole) and strongly recommend them to others. Since going to AMSOIL, I check the color and levels quarterly. To date (well over a year), the AMSOIL gear oil is still golden colored and is working like a charm.

To be honest, since that pumpkin nightmare and my AMSOIL trial(s), I have switched both vehicles over completely to AMSOIL. I guess I would be considered an AMSOIL Kool-Aid drinker, but I've tried several products and am sold completely.

Some testing of gear oils :

http://www.amsoil.com/products/gearl...hitePaper.aspx

http://www.amsoil.com/lit/g2457.pdf

Hope that helps !!
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