Any preference for these 75w90 Synthetics???

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mercuryblues

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I noticed that the rear end cover is leaking on my Grand Marquis (it was only sealed with RTV on the last service and no gasket was used, not the best idea... I know). Since a new gasket will be going on, I'd like to stick with a 75w90 synthetic as I'm looking for an extended drain interval. The car has 193k miles now, but it is running very strongly and I'm hoping to go another 75k-100k+ with it. In fact, I'm hoping to never touch it again after the new gasket goes on.

Mobil 1 is in the car right now, but I've also been looking at Castrol Syntrax and Valvoline Synpower. So even though this may not matter much, I'm curious to see if you folks if you have a preference (and perhaps a ranking) amongst these three. The other factor is that I don't have a limited slip (2.73 ratio and axle code C8), so there could be other ideas presented here, which I'm very open to. Based on everything I mentioned, I hope it doesn't make the whole post a total waste but I definitely appreciate the feedback, none-the-less. Thanks everyone.
 
Originally Posted By: mercuryblues
I noticed that the rear end cover is leaking on my Grand Marquis (it was only sealed with RTV on the last service and no gasket was used, not the best idea... I know). Since a new gasket will be going on, I'd like to stick with a 75w90 synthetic as I'm looking for an extended drain interval. The car has 193k miles now, but it is running very strongly and I'm hoping to go another 75k-100k+ with it. In fact, I'm hoping to never touch it again after the new gasket goes on.

Mobil 1 is in the car right now, but I've also been looking at Castrol Syntrax and Valvoline Synpower. So even though this may not matter much, I'm curious to see if you folks if you have a preference (and perhaps a ranking) amongst these three. The other factor is that I don't have a limited slip (2.73 ratio and axle code C8), so there could be other ideas presented here, which I'm very open to. Based on everything I mentioned, I hope it doesn't make the whole post a total waste but I definitely appreciate the feedback, none-the-less. Thanks everyone.

Amsoil SVG
 
I went with Genuine GM 75W-90 in mine. There was a tag on the differential that said Synthetic 75W-140, which is what mine was factory filled with. But I don't tow or drive at constantly high speeds, etc. I have gained some MPG switching to 75W-90 though.
 
I used M1 75-90 in my 96 Merc GM. It had 218K when I sold it and I only changed the lub only once.
 
Can't go wrong with the M1, it's good stuff. I just changed mine in my truck, which requires synthetic 75W-140. I had planned to use M1, until I saw that AZ had Synpower for $12.99/quart vs $20.99/quart for the M1. Don't know why the M1 75W-140 is so expensive, because in 75W-90 the M1 was only $1 more than the Synpower.
 
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have used amsoil, mobil 1 and valoline. to be honest I've never been able to tell any difference in any of them. The only non-rational impression I have was that I had one noisy diff that was a little quieter with mobil than it was with valvoline.

I now run amsoil because I can get it, but I think M1 makes a fine diff fluid.

M
 
I too have used with great success, the Mobil 1 75/90 LS in many units. IIRC, Advance Auto Parts has a decent price on M1 75/90 LS @ $10.99/qt.
 
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