Due to something coming up I will not be able to change drive shaft u-joints as planned.Can I use synthetic grease in the u-joint if regular grease has been used earlier?I want to grease them before I drive any real distances,they are not making any noise I changing them because I did the half shafts and just want all new for piece of mind.So is it ok to mix syntheic and regular grease?

It won't hurt a thing as long as you are buying a synthetic bearing grease.

I used the Mobil one syntheic stuff..I am sure whatever was in there was not synthetic but I know you can mix it without a problem..I found it at auto zone..I put a lot in there and drove it a few days and did it again to make sure all the caps got grease. Glad to hear you got the half shafts done and got rid of that u joint noise.!.
Rich