I changed the oil 300 miles ago. Right afterwards, I noticed that
the dipstick was out of its hole, and that the engine had coated the
surrounding area with a fine mist of oil. I cleaned things up and
stuck the stick back in the hole.
.
Today, I noticed that the same thing had happened again. I replaced
the o-ring on the dipstick and stuck it back in the hole. I ran
the engine at various RPMs with the dipstick out, and no blowby
came out of the tube. The oil cap (plastic) jiggles a bit at idle
when set on top of the filler, but if I push it down briefly to
make the seal grip, it sucks down tight. The flame trap was
cleaned recently, and the car doesn't leak oil from anywhere else.
.
Could the dipstick have vibrated out of the hole? The o-ring was
old and dried out - it barely held it in the tube. On a car in good
shape, if you drive it with the dipstick out, should some oil mist
rise up out of the tube?
.
Maybe I was careless and didn't push the dipstick in well both times.
I did check my oil yesterday, so carelessness is a possibility.
.
TIA -b.

That popped out dipstick and oily mess happened twice on my '88 240. I cleaned the flame trap and orifice on the maifold carefully the first time and the stick still came out. I replaced the o-ring and so far it is holding. Like yours, my old o-ring was like a hard piece of plastic. The new one feels tighter.

When I do the jiggle test, the oil filler cap stays put even though it is the lighter plastic one.

If the new o-ring solved the problem and you recently cleaned the flame trap, and there are no other oil leaks, then there's probably no concern about crankcase ventilation.

By the way, the oil cap test doesn't work with the plastic caps. They're too light and will skitter around no matter what.

If after replacing the O-ring you still have the problem or other oil leaks develop, you need to take apart your flame trap and oil breather to really check it good. It's possible that even though the flame trap was cleaned, some of the associated vacuum lines, or the oil separator box, are obstructed.

I think that I've found the cause of the problem. The oil was
overfull by about a quart. Probably my oversight combined with gas
in the oil from when the motor was skipping due to the plug wires
shorting to the block. (Crosses fingers.)
.
-b.