What engine oil should I get for an '88 GMC S15 pickup?

The engine has minor oil leaks, probably because it is old and has greater tolerances than newer rigs. Also, the temperature where I live can be as cold as 10�F in winter and as hot as 90�F in summer. I've also noticed there might be a small amount of sludge buildup. The previous owner used 10w30, so that is what I have been using, but what oil would be ideal for my truck? Also should I switch oils between seasons?