My 1972 CB750K2 just turned over 42K miles and is still going very strong with no leaks or unusual noises. It even has the original clutch, though I think I might change that at 50K, just to keep everything up to speed. I adjust the valves and keep everything tight on the usual intervals, and always use 20W50 motorcycle oil.
I have ridden it all over North America, through Death Valley at 115 degrees, across the Grand Tetons in freezing rain at 10,000 ft., and it has never quit on me. I live in Atlanta, Georgia now, commuting everyday to work through traffic with temps around 85 degrees, and still no trouble (knock on polished aluminium)! Here's to 100K!