Hello, Tony,
10w-30 or 10w-40, or 15w30 or 15w40 (I like Rotella T oil)
No need for synthetic oil.
The only concerns about the switch-over from older non-detergent oil to modern detergent oil is that detergent oil keeps more "junk" in circulation (to be removed by an oil filter - if you have one). Non-detergent oils tend to allow "junk" to drop out in the oil pan. There is a possibility that after switching to detergent oil, some of that "junk" will go back into solution.
Some people advise you to remove the oil pan and clean it out (and clean the oil pump "pickup" screen) when switching over from non-detergent to detergent oil. Of course, if you have one, change the oil filter (and, canister) at every oil change.
