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This subject has probably been flogged to death, but I need your comments. Here in Canada, the GM 75W-90 gear oil costs C$30.00 per litre (near enough a US Quart). It complies with this mysterious GM specification 9986115 which seems to exclude any other make of oil under the sun. The cost of Castrol, Shell, Mobil 1 etc synthetic GL-5 75W-90 is about C$12.50. Apart from gouging the customer, what is so special about 9986115 that justifies the cost differential.


If I use the non GM oil, will it nullify my warranty if I have problems? What are your recommendations for synthetics for both differentials?
 

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I'm running Mobil Synthetic..... I refuse to pay them high way robbery prices ....

Mobil

Mac Edited by: Mackin
 

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Amen Mac !!! I agree 100%. Like I have said before....Its gear lube...NOT Holy water !!
 

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Double check to see if your truck's front diff came with synthetic factory or if the dealer replaced it. There's a TSB out to cover the change in cold climates. My dealer in MN did it no questions. No sense in wasting time and money if the General will do it for free.
 

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The TSB encompasses not only a switch to synthetic, but a new breather vent cap. I swapped the cap on my truck, because there is some "issue" with the stock cap and synthetic fluids.

I run non-GM synthetic in my diffs (Amsoil), and have yet to have any problems. It's my understanding that the GM stuff isn't anything special.
 

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I just changed all my gear lube and went with Mobil 1 syn. According to dealer and service manager as long as the lube meets GL-5 standards and is syn. where called for, I will not void my warranty. Service manager even stated that the last delership he worked at stocked and used mobil 1 products.
 

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RedLine.... at your local speed shop. About $10 a quart.

 

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Read the label.... GL5 perfect and pure synthetic....


Edited by: hoot
 
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