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Fluid change - 115k miles


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Question for all interested...

 

My 2006 Silverado Z71 is due for transfer case and differential fluid changes. The last time the change was done, I was charged extra for "special" rear gear oil that GM recommends. This time, I plan on doing the change myself. The rear shouldn't be any big deal using 75w-90 synthetic. Do I need to get the oil with limited slip additive for the locker rear?

 

For the front, I'm assuming it was replaced last time with 80w90 gear oil. Will I experience any future leaks if I switch over to 75w-90 synthetic since the synthetic oils obviously have a different make up? I read on other forums that at higher mileage, I should stick with what it's had so seals and gaskets won't leak.

 

Lastly, I read that I should fill the diffs till fluid starts flowing from the fill hole. I also read some people say that you should fill to 1/2" below the fill plug hole. Does it matter based on the type of oil used?

 

I know I need 2 qts of transfer case oil, about 2 qts of front diff oil, but how much for the rear? Thanks all.

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No need to additive for rear end. Fill the rear to about 5/8" to 1-5/8" below filler hole. Syntheic is what you need for the rear. I can tell you all this because I just went through this with mine just a few months ago. For the front 75w-90 will be fine, I used the 80w-90 because I was cheap. You will need about 2 quarts for each diff, 4 quarts total. Now for transfer case, if you have Auto 4wd make sure to get the Autotrak 2 fluid. If not Auto 4wd, I think you need Dexron 3, but not sure

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All you need is a GL5 rated synthetic oil, the GM spec is for a GL5 gear lube.

 

 

What ever you do DO NOT use a Limited Slip Additive in a locking rear differential. The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.

 

fwd_lubes.jpg

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Remeber to loosen the fill plugs before draining the fluid. It's a real pain when you've drained all of your fluid, and then realize you can't break loose the fill plugs.
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Fill the rear to about 5/8" to 1-5/8" below filler hole.

 

 

Sooooo, how do I know when it's about an inch below the filler hole? What happens if I fill it until fluid comes out the filler hole? I purchased 3 quarts of Lucas 75w-90 gear oil for the rear and 2 quarts of Lucas 80w-90 for the front. The reason I didn't buy the 75w-90 for all of it is because the Advance Auto Parts didn't have 5 quarts of the same. I still need to get the 2 quarts of Auto Trak II. I didn't know this wasn't available at the store. Oh well.

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I used a pipe cleaner that are used in crafts and bend it so I can use it as a dipstick for the diff. You can also use an allen wrench. Just make sure the bent portion is long enough to reach to the correct level

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I used a pipe cleaner that are used in crafts and bend it so I can use it as a dipstick for the diff. You can also use an allen wrench. Just make sure the bent portion is long enough to reach to the correct level

 

Dang I wish I had thought of that. I didn't have an allen wrench to use so I used some safety wire, I doubled it over and put a twist on it so I could "see" what the level was.

 

Scott, if you leave your rear diff too full of fluid it will start spilling out through grease seals at the ends of your axle and then you will have fluid leaking into your braking area. I only know this because I found out the hard way on my old rust bucket '76 Chevy years ago.

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  • 2 weeks later...

Ok. I finally got all the fluids swapped out. I hope I did this correctly. The transfer case was simple enough and I've done that before, so it was no big deal. I do have a couple questions on the front and rear diff though. I read, just today, that the front diff should be 1/2" below the fill plug when cold and to the bottom of the fill hole when at operating temp. Will I have this spillover issue on the front if I filled it to the bottom of the fill hole while the truck was cold?

 

On the rear, I found my truck was about 5/8" below the fill plug after only two quarts. Everything I read said that I would need to buy three quarts because I will need a little over two. I never needed to open the third. Should I drive around and distribute the fluid, then check it again? I checked it 3x to verify I wasn't getting random fluid on the allen wrench tip.

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