I'm wondering what the specific factors are that cause oil seals to leak.

All of the following cars use 10W30 dino oil and are meticulously cared for (preventive maintenance is standard, except that I don't replace oil seals unless they leak).

I have a 1980 242DL with nearly 300k. Its seals were changed in 1994 in Seattle. I bought the car 3 years ago with a dirty flame trap and blown head gasket. I've never leaked a drop of oil.

However, I've owned 2 late model bricks (1985 and 1989) with < 180k miles on odometer. Flame traps were dirty, but no major engine problems. With both cars, one year into my ownership, the camshafts leaked badly. I put maybe 10k miles of city/freeway driving in San Francisco, no different from my 1980 242DL.

I pamper my cars, so clogged flame traps are not reasonable explanations.