I have a recent purchased '87 wolfsburg weekender with 98K on original engine.  No leaks, runs good.  Avg MPG on highway is 18+.  Previous owner used Mobil 1 Synthetic 10W30 which I keep in it also.   I drive it daily but concerned that oil may be too thin for hot weather in SE.  (and it is damn hot right now)  Someone told me the synthetic oil also retains more heat than regular oil.   Thanks for your insights.  Steve