Jim; The engine oil drain plug is on the left side of the pan. It is a hex head plug, NOT the flush mount allen head next to it. The transmission drain is close to the right frame rail. The primary drain is usually just forward of the bottom of the derby. Careful, this one is sometimes a tapered pipe fitting.
Oil quantities are usually 32 oz. transmission, 32-38 oz, primary (best to fill just to bottom of clutch basket when standing straight up. I don't remember if the '88 were 3 1/2 or 4qt. engine oil. Use dipstick.
I'm not gonna get into the "which oil is best" but I recommend a synthetic air cooled 4 stroke 20w50 oil for the engine. A quality 75-90 or 75-140 gear oil (synthetic is worth the $$),for the trans, and Harley formula + for the primary.
HD oil filter for twin cams are; 63798-99A(chrome) , 63731-99A(black). If your are going to the HD store for filter, also recommend picking up new o-rings for the drain plugs.
All this data is in the owners manual if he has one.