I had a guy rebuild my rear end but when I looked at it when it was done it the posi unit was covered in brand new black grease. Is that normal or will it affect any. Now I know it needs gear oil after I put the cover back on but what about the grease?

 

He might have used the grease to determine the contact patch. I don't why black but it shouldn't hurt anything. Once the rear is on the road, the grease will mix with the gear oil and all will be well.

 

For the contact path he used the stuff provided with the new gears I bought but it was yellow. Anyways, I just want to make sure the grease would not affect it any because this the first time I've heard or seen something like that.

 

many places use grease during assembly to insure initial lubrication. Until those parts have been in fluid and the pores have soaked up some lube, its a good idea. - basically the same as installing a cam.