Rear Diff Fluid ????????????
I have run the Lucas 75w90 or 80w90 in the past just because it does not require you to add an additional lubricant for the clutch type posi. Now that I have a Torsen differential I just run straight Valvoline with no additive.
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Are you looking into changing the oil that's currently in there? If so, do you know what's in there now? I had a discussion with an oil company tech rep a number of years ago, and his comment was that this company didn't like to see syn oil go into a rear end that had over 50K miles with regular gear lube.
This wasn't because there'd be an issue with the gears, and clutches, if it's a posi, rather the seals had become accustomed to the additives in the regular oil, and the different chemical make up of the syn oil could cause the seals to leak.
Of course I had this discussion AFTER I had put syn oil into the 70,000 mile rear end in one of the "family fleet". The rear was a posi, and I was getting a little chatter afterward. I called the oil company's tech line to inquire about the posi additive, and the discussion went on from there. As it turned out, the rear didn't show any leakage, but then we wound up selling the car about a year and a half later, so I don't know what happened long term.
This wasn't because there'd be an issue with the gears, and clutches, if it's a posi, rather the seals had become accustomed to the additives in the regular oil, and the different chemical make up of the syn oil could cause the seals to leak.
Of course I had this discussion AFTER I had put syn oil into the 70,000 mile rear end in one of the "family fleet". The rear was a posi, and I was getting a little chatter afterward. I called the oil company's tech line to inquire about the posi additive, and the discussion went on from there. As it turned out, the rear didn't show any leakage, but then we wound up selling the car about a year and a half later, so I don't know what happened long term.
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I just recently changed my rear-end fluid. I used Royal Purple 75w-90. The bottle says it includes limited slip additive. I didn't add anything but the Royal Purple and I have ZERO whine/noise from the rear-end. Less than I had to begin with 
. Although I am afraid I didn't clean the gasket surface good enough because I now have a slight leak. Be sure to THOROUGHLY scrape the gasket surfaces clean when you are changing the fluid.
. Although I am afraid I didn't clean the gasket surface good enough because I now have a slight leak. Be sure to THOROUGHLY scrape the gasket surfaces clean when you are changing the fluid. 
I just recently changed my rear-end fluid. I used Royal Purple 75w-90. The bottle says it includes limited slip additive. I didn't add anything but the Royal Purple and I have ZERO whine/noise from the rear-end. Less than I had to begin with . Although I am afraid I didn't clean the gasket surface good enough because I now have a slight leak. Be sure to THOROUGHLY scrape the gasket surfaces clean when you are changing the fluid.
. Although I am afraid I didn't clean the gasket surface good enough because I now have a slight leak. Be sure to THOROUGHLY scrape the gasket surfaces clean when you are changing the fluid. One other thing...If you opt to pull the cover off the rear to drain the fluid, as well as carefully scraping the old gasket off, make sure to get the oil out of the lower threaded holes on the rear end housing.
As the oil drains out of the rear, a certain amount of it will "backflow" into one or two of the bolt holes. You then install your newly cleaned rear cover, and your new gasket, and as you thread in the bolts, this residual oil is squeezed out, soaking the gasket material adjacent to the bolt holes, and you have a leak. Trust me, I've been there......
As the oil drains out of the rear, a certain amount of it will "backflow" into one or two of the bolt holes. You then install your newly cleaned rear cover, and your new gasket, and as you thread in the bolts, this residual oil is squeezed out, soaking the gasket material adjacent to the bolt holes, and you have a leak. Trust me, I've been there......
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Joined: Jul 2008
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From: Miles City Mt
Hope you do not mind, I am not trying to high jack your thread or anything. But since we have similar questions I thought it would be better to post here. What fluid do you guys recommend for a rear end with these mods? I want to get your guy's input on this
-Moser 12 bolt rear end with larger 33 spline axles, full spool, upgraded performance cover with main cap studs, 4.10 gears, and a 1350 yoke.
-Moser 12 bolt rear end with larger 33 spline axles, full spool, upgraded performance cover with main cap studs, 4.10 gears, and a 1350 yoke.
