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Discussion Starter · #1 ·
Hey All,

I've dug around a bit here, but haven't come across a definitive opinion of the oil to use for the rear end. Redline 75w90NS is the one more often referred to, but the friction modifier for a LSD is way less clear. As I think about scooping up oil to change out my rear end oil, I'd love to know if the Redline 75w90NS is good with or without an additional friction modifier.

Further, if a friction modifier is used, what are people buying? I ask because it looks like Ford & GM have their own OEM additives available but not sure if they work well, or if there is something else that would be recommended.

Thanks!

Joel
 

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Might depend on what car you are driving. LS additives are not needed for non LSD differentials but they won't hurt
 

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Discussion Starter · #3 ·
To add more information here, car is a '73 Spider 2000 which sounds like it should have a LSD though I have not confirmed this myself.
 

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NS doesn't have LSD friction modifier. This will increase LSD lock up but may cause chattering if the LSD locks up too much,

You can just use the NS and see how it goes. If it chatters (which is probably unlikely) you can add a little friction modifier until it stops.
 

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I'm running conventional 80w90 Castrol gear oil that can be found at CT and/or PartSource.
When I bought my car 10 years ago, I switched to Redline in both the trans and diff, and very soon after both started leaking very noticeably. If synthetic was already in there, you should be fine with Redline, or perhaps if your seals are nowhere near original. I say this because most say that leaks start due to the switch to synthetics is because of different detergents that can wash away crud build up from the conventional oil helping to seal old seals, that gets cleaned away.
I'm sure this isn't everybody's experience was mine and I'm sure some others.

Do you know what is currently in there or any chance you are able to ask the prior owner or shop they used to find out what's currently in there?
 

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you can easily see if your diff has a LSD
look for the triangular casting
1620218122088.png
shoulkd you want a friction modifier, might as well get one from Redline, if you are ordering their oil.
 

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So just to be clear, the consensus is that 75w90NS is safe to use in the diff?
 
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