Front Diff Fluid?

cujo8 · July 23, 2009

I have a 2006 GMC Sierra K1500 and the manual calls for a standard SAE 80W-90 gear oil (non-synthetic), but I would like to use a full synthetic gear oil in the front axle if possible. Do you see any problems using synthetic gear oil in the front axle? How closely do you need to match the gear oil viscosity of 80W-90? What are you guys using in your front axles? Thanks.

Bish · July 23, 2009

I've run Mobil 1 or Amsoil front and rear since my 2001 Tahoe with no issues in over 250K miles.

DJinCO · July 23, 2009

Ditto.

Z71Silverado · July 23, 2009

you don't use it enough to warrant synthetic gear oil

rogerf · July 23, 2009

I have a 2006 GMC Sierra K1500 and the manual calls for a standard SAE 80W-90 gear oil (non-synthetic), but I would like to use a full synthetic gear oil in the front axle if possible. Do you see any problems using synthetic gear oil in the front axle? How closely do you need to match the gear oil viscosity of 80W-90? What are you guys using in your front axles? Thanks.

No problem. 75W-90 is the synthetic replacement for 80W-90 dino-oil and will run the same stuff in both axles when the time comes.

jf781 · July 23, 2009

I remember reading somewhere on hear that you can use synthetic in the rear diff but the front your supposed to stick with the dino 80W-90. There were some seals that did agree with synthetic if I remember right.

Matt (sk1er18) had some good information on this and regarding the use of a gasket vs. RTV for the rear diff (short answer - use a gasket and RTV). Hopefully he will chime in shortly but I am going stick with dino up front and mobil 1 in the rear.

Matt_ · July 23, 2009

Synthetic is not recommended for the front because 1) the front isn't used enough to warrant the benefits and 2) synthetic fluids are known to break down the seal and lead to leaks on the front diffs (gaskets are used on the rear).

Can you run synthetic? Sure... But there's no benefit, its more expensive, and it could result in a leaking diff....

Edit: I can't find the thread because the search never works right on my phone, but there is a thread where I had a lengthy "discussion" about the problems with synthetic fluids and RTV gaskets. Two of the people who said I was full of crap ended up finding leaks and changed to a gasket, and another person actually found a tsb from GM stating an issue with running synthetic fluid in the front due to breaking down the seal. So my recommendation is to stick with 80w-90 dino up front

Bish · July 23, 2009

In an awd, don't you use the front just as much as the rear? I know they had an issue with vent tubes about 8-10 years ago and if you used the synthetic you could get leaks. I believe over the past 5 years or so the front diffs are now compatible with synthetic. I look at synthetic as cheap insurance.

Matt_ · July 23, 2009

Yes AWD is a completely different beast. My posts were assuming a normal 4wd truck.

Some newer models are compatible with synthetic, but I don't think an 06 is, and as mentioned, for 4wd there really is no benefit IMO so I'd stick with the recommendation on the owners manual of non-synthetic.

Z71Silverado · July 23, 2009

In an awd, don't you use the front just as much as the rear? I know they had an issue with vent tubes about 8-10 years ago and if you used the synthetic you could get leaks. I believe over the past 5 years or so the front diffs are now compatible with synthetic. I look at synthetic as cheap insurance.

i used synthetic in my front diff for a period of one winter. my front diff started leaking(i was finding small drops on my driveway). i then switched back to normal diff fluid in the spring and the leaking has stopped. of course with awd vehicle it's a totally different topic. the question was......what should he use in a 4 wheel drive pick-up truck. not an awd drive vehicle.

cujo8 · July 23, 2009

Thanks for all of your input guys. You've convinced me to stick with conventional SAE 80W-90 in my front diff. Thanks.

gave20 · July 23, 2009

I run synthetic in both diffs and never had a problem.

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rgrant · July 23, 2009

Sorry, but the statement that synthetics will break down "seals" is a myth. Synthetics and dinos all have additive packages that include seal conditioners to control seal swell, shrinkage, and hardening. 2004 and older front diffs have a different vent style and are not recommended to use synthetic unless upgrading to the newer vent. Any 2005+ will be just fine with a synthetic GL-5 oil in the front diff. Personally, I prefer synthetic because it's easier to buy all the same oil instead of stocking 2 types. If you purchased a 1500HD you would have the EXACT same front diff as you would in a regular 1500, except it would be filled with GM's synthetic "grape juice" too.

Long story short, either are just fine.

Matt_ · July 23, 2009

I wouldn't say it's a myth.. it's been substantiated by tsb's, i've seen plenty of diffs leak due to RTV seals breaking down from synthetics, and members from this board have checked theirs to find leaks.

From a previous thread (sorry Ryan, hope you don't mind me using your quote)

I've never had a leak.......... until now.... Changed my rear diff oil a month ago and used M1 75W-90 Syn and black RTV to seal. After thinking about this thread I peeked under the truck yesterday and damned if there was not an oil spot on the garage floor. I cleaned everything good and let the RTV dry a good two hours before filling. It's been almost a month. I'm going to tighten the bolts a tad tomrrow and if it still is dripping I will be putting on a gasket. I put my foot in my mouth before as this is the first one I've ever had to leak using RTV and Syn.

I'll see if I can find the TSB# where GM advises their tech's not to use synthetic in the front diff due to the seals breaking down. I remember reading it, might have been while I was at the shop though.. we'll see if google works..

Will using synthetics 100% cause a leak if using RTV? No, of course not. Can it? Yes.

Edit: I found this but can't get the TSB to read, it might be it: 2004 Drivetrain - Synthetic Fluid Usage in Front Differential 20040901 <-- can someone check that number to see what it says?

cujo8 · July 23, 2009

I was going to ask another question, but I got mesmerized by gave20's avatar.......................

Oh, now I remember what I was going to ask, If I have 3.42 gears in the rear diff, I assume I have 3.42 gears in the front as well correct? Is the front also a posi-differential? Also, I'm assuming when folks do a gear swap in a 4x4, they have to swap both the front and rear gears on a 4x4 correct. Thanks.

Now back to that avatar.........