RTV Sealant and Synthetics?

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I just got off of the Amsoil Online Product Applications Guide and found that they claim synthetic oils will eat RTV sealant in applications such as the rear differential. Is there any merit to this? I was planning a fill of Lucas synthetic 80W-90 and use RTV to seal the rear. Is this not a good idea? Should I buy a gasket instead? I have always used RTV and never had a problem, but I want to make sure I prevent one in the future. I also read about rears with high pinions not being properly lubricated by synthetic gear oils. Would I be better off to use a product such as Penzoil or Supertec and use the Lucas in a field mower gear box or something?


Also, I am going to do a simple drain/ refill of the transmission with Valvoline Maxlife. The guide says the initial fill is 5 Quarts. Is this how much I will replace with a drain and refill? The vehicle in question is a 1996 Chevy Silverado K1500. I thank you for your help!
 
These notes take a little interpretation.

Quote:
Differential, Front......GL-5
ALL TEMPS......80W-90 [5]


Quote:
[5] 1988-99 models
Do not use synthetic lubricant in axles using RTV sealant.
Synthetic lubricant will chemically attack RTV sealant.
American Axle has only approved the use of Synthetic
Lubricant 75W-90 GM part no. 12378261, as follows:
a. All 8.6 inch rear axles are compatible.
b. All 9.5 inch rear axles built before mid-February 1998
(RTV sealant used to seal cover pan) are not compatible
c. All 9.5 inch rear axles built after mid-February 1998
(equipped with cover pan & gasket) are compatible.
d. All 10.5 inch rear axles built prior to March 3, 1999 are
compatible if axle shafthub RTV sealer is replaced with GM
gasket part no. 327739.


First of all, Amsoil gets the data from All Data who get it from American Axle, with this only applying to the above applications. So it's specific to American Axle and whatever RTV they used. Not sure which front/rear diffs you have but if you replace the RTV with known good RTV, there will be no problem with synthetic gear oil.
 
Manufacturers often use silicon to seal the diff housing,so bit of a sweeping statement.They most likely mean the vinegar smelling silicons,solvents degrade them - OE silicons won't be affected.
 
Originally Posted By: bdcardinal
if that were true, every Ford that comes with 75w140 from the factory would leak.


x2. Lots of vehicles leave the factory with synthetic and RTV with no problems.

Currently I have Royal Purple Max Gear in my differential with Permatex Ultra Black sealing the diff cover, no leaks yet.
 
Originally Posted By: 01rangerxl
Originally Posted By: bdcardinal
if that were true, every Ford that comes with 75w140 from the factory would leak.


x2. Lots of vehicles leave the factory with synthetic and RTV with no problems.

Currently I have Royal Purple Max Gear in my differential with Permatex Ultra Black sealing the diff cover, no leaks yet.

I have the same combination on my mom's car.
3 years and 50,000 miles later, it is still perfectly sealed.
 
Factory fill Motorcraft 75W-140 Synthetic with Motorcraft TA-29 (ultra gray) RTV in the F150, zero leaks in almost 140,000 miles.
Redline 75W-140 with Motorcraft TA-31 (high temp gray) RTV in the F150, zero leaks.
Redline 75W-90 with Motorcraft TA-32 (clear) RTV in the Mustang, zero leaks.
 
RTV leaks are generally due to the installer.
A cruddy aquarium clear type RTV is not suitable for automotive use.
Most RTVs are labeled if they are suitable.
 
Originally Posted By: mechtech2
RTV leaks are generally due to the installer.
A cruddy aquarium clear type RTV is not suitable for automotive use.
Most RTVs are labeled if they are suitable.


So I wonder what kind of RTV American Axle used.
 
Originally Posted By: mechtech2
A cruddy aquarium clear type RTV is not suitable for automotive use.


Ford calls for clear silicon RTV (Motorcraft TA-32) on the diff cover in several apps. My buddy, a Ford double senior master, uses it for everything. 1 hour dry time and no comebacks due to leaks. That clear RTV is perfectly suitable for automotive use.

I've started using TA-31 on diff covers, which is designed for high temps and fast cure time. When I swapped to Redline in the F150 I gave it about 1.5 hour cure time...zero leaks and it clamps better than the clear.
 
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