2001 Dodge Ram 2WD limited slip - synthetic yes/no? Additive already in the oil?

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Nov 11, 2020
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Ontario, Canada
Vehicle is 2001 Dodge Ram, 64k miles, 2WD, Limited Slip differential. Planning on changing the differential oil. I have Pennzoil Platinum 75W-140 Limited Slip full synthetic and Castrol 80W-90 limited slip (non synthetic). The manual says 80w-90, and also says for heavy use the 75W-140 synthetic is recommended. My questions:

1) both oils say limited slip on the label, and I think they mention additives in the fine print. Is there any reason I should not trust these as having the right and proper amount of FM additive? (general rule seems to be to buy oil, buy additive, and mix, but why since they sell oil that seems to have additive already in it?).

2) I don't use the truck for heavy-duty service, but I assume I can still use the 75W-140 synthetic (unless maybe thats not really the best choice for light-duty use?). Another aspect here is -> will changing to synthetic mess up the original seals? Lead to leaks?
 
Joined
Aug 2, 2018
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I don’t see any issue using 75w140 synthetic “may or may not” require the friction modifier.. it’s kind of a foolproof move. The 80w90 may say limited slip use but more as a top off fluid.
 
Joined
Jun 28, 2012
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Southern Illinois
Won't hurt a thing. "If" you get some clutch chatter, add some friction modifier 1oz at a time until clutch chatter is gone. Go do a backwards figure 8 a couple of times to work new oil into the posi clutches. But I doubt you'll need anymore FM.
 

SumGuy

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Joined
Nov 11, 2020
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Ontario, Canada
Regarding synthetic oil maybe causing leaks, I found this (about Toyota Tundra circa 2001-2003):

Rear axle seal leaks caused by syn. gear oil (at https://www.tundrasolutions.com/threads/rear-axle-seal-leaks-caused-by-syn-gear-oil.6964/ )

"A note in DL003-03 states that "Toyota Genuine Synthetic Differential Gear Oil (P/N 00289–75140) CANNOT be used in 2000–2002 Tundra vehicles and 2001–2002 Sequoia vehicles. The rear axle oil seals are not compatible and will leak if synthetic differential gear oils are used in these vehicles."

So this was an issue for some vehicles back during this time frame.
 
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