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Not sure why you're asking. I mess with older cars but last time I checked the fluid of choice was 90 wt gear lube. Don't know if they make that synthetic or not. As a rule of thumb if the car started life on synthetic then leave it if it stared on regular oil then leave it. I'm sure others will have other opinions. But last I heard motor oil stays in the motor, gear lube goes in the differential.

Mike
 

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Gear oil, yes. Make sure it's marked "80W-90", or plain "90 weight". If it's one of the more expensive synthetic versions "75w-140" is usual. Motor oil isn't thick enough for use in rear differentials.
 

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I use synthetic blend 80-90WT because I'm allergic to the smell of dino gear oil. The bit about "if it stared on regular oil then leave it" is pure BS, or I would have puked my brains out long ago.
 
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