The last internal engine problem I had was with a 200cid (3.2 liter) inline six from Ford - the pistons had to be de-carboned after a cross-country drive. This was in 1985, so I blame the 55 mph national speed limit for not letting the engine run harder. Ever since - I just do 5000 mile oil changes and 3 year coolant changes, and all is well. They're really quite reliable these days, even as the complexity has gone up with turbos, direct injection, variable valve timing, and so on.