I've never seen so much foam, ever. The dipstick tube was choked with the
stuff. (background -- the temps were cold, like 10-20F, and I had driven
only a couple blocks to the gas station). I couldn't read the oil level on
the stick, there was so much foam. For a moment, I thought that ALL the oil
in my pan had turned to foam but realized, after repeated attempts at reading
the stick, that the tube was full of foam and covered the markings when I pulled the stick out.

Basically, I had to take it on faith that there WAS oil left in the engine
(due to the fact that there was no low oil pressure lights/warnings) and let
the engine heat it up to liquid form again. It was ugly.

I forget the weight I'm using: 10W-40, I think. maybe 10W-30. ...whatever
the manual recommended is what I put in there.