Rear differential oil question

65GUS

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Looking to change out the oil in rear diff in my 65 fury 3. The quickie lube has a powered suction tube to remove old oil, but carries only newer synthetic gear lubes like 85-90 & 85-140 wt oil. Is it OK to use the newer synthetic ??? Or should I buy bottles of convention gear lubes ?? Thanks
 
I don't if I have a sure grip or not, how do I identify that ??
 
You most likely have an open diff. Jack up the rear and spin the tires to find out for sure.

I run synthetic in my rear ends.
 
The synthetics are overkill in an 8.75" rear end, but they won't hurt anything. Do not run the synthetic with a sure grip. If you have a sure grip, both wheels will turn the same way when turning one rear wheel or the other.

Dave
 
What seems a bit interesting is that after GM went to syn rear axle lubes, they continue to recommend changing it every so often in the 2500HD pickups, which apparently is necessary for best longevity.

All of the prior-gen rear axles (i.e., but not limited to, Chrysler 8.75", Chry 9.25", Ford 9", Chevy 10-bolt and 12-bolt) all lasted just fine with the normal 90W axle grease in them. Just getting things topped-off every so often during an oil change. HD Trailer or other severe use service schedule probably did recommend changes at some mileage interval, but the normal daily-use cars almost never got any rear axle lube changes for well past 100K miles, if then. Of course, suctioning it out and putting in fresh lube is probably good, too, especially if the car is used in colder climates and is driven short distances each time it goes out. Just as with motor oil, the oil needs to get hot enough for a long enough time to cook-out the accumulated moisture/condensate in the rear axle assy and its lube . . . for optimal results . . . regardless of whether it's "dino" or "syn".

Just as with motor oil, heavier gear oil absorbs more horsepower than the normal (85W-90) viscosity will.

Just some observations,
CBODY67
 
I used the Lucas 80w-90 before but with a newly built sure grip am using Supertech 75w-90 with additive. This is semi-synthetic and the diff builder ok'd it. Something about not recommending full synth but a blend lets you top it off with anything later on. When I change it again I'll confirm again with the builder.
 
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