When I had the gears installed in the rear diff 4.56s they used regular gear oil instead of synethic its been about 15k so I would like to change oil can I just drain and go back to synethic ?

 

Yes you can.I suggest royal pruple for cost and it seems to be a pretty good oil.$10 a court at pep boys if you have one by you.other options are mobile1 for $16 a coourt at autozone or the motorcraft brand from ford for $20 a court.you need 3.5 quarts I believe.FYI,you should have changed it 500 miles after putting in the new gears I believe.