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Gear oil

If I had a rear differential that had synthetic gear oil in it and i topped it of with regular gear oil would that cause a problem? and is there special fluid for a ford limited slip differential?

I think most limited-slip diffs take an additive that helps the clutch work right. Mixing engine oils of the same weight is not a problem; I’m not sure about gear oil, though I suspect it would be fine as long as both oils are the same weight.

What does your manual say about the requirements?

It might make a difference. Do you have an owner’s manual? It would list the requirements. So could a Ford Dealer.

its just for generic reference, it is from a discussion i at work had. I have always used any 80w90 that was available at the time. I was told i was causing damage to my vehicles. The guy i was talking to also mentioned something about only using fords special differental fluid, and the only reason i could think of using special fluid would be limited slip differentials.

The answer to your question is different for different cars and models of cars.  You need to take a look at the owner's manual where you should find the answer to your question.  Some are very specific and may only be available from the dealer and others may be general and available at WallyWorld.

For a limited slip diff I would only use the specified oil, the clutches could react to incorrect oil. So yes, it could make a difference.

If you’re just topping it off then it probably won’t make much of a difference. My last 3 SUV’s all had limited slip. I’ve replaced the differential fluid in all of them. Switched to synthetic which didn’t require the additive. If I used non-synthetic then I’d have to buy an ounce of the limited slip differential additive. So I seriously doubt that just topping off with regular gear oil is going to make a difference. You can always buy the additive if you want…it’s cheap.