I get my thermostat out (on the same engine) by removing the whole
housing (I have to disconnect all the hoses) and tapping it out from the
"down" side. There's a brass sleeve that sits on top of the thermostat
that should just slide out (it just slides in) but never does of
course. You could probably just grab the old thermostat with vise-grips
or channel locks and yank hard but I don't mind taking the whole
kit-and-kaboodle off once in a while. You also need another gasket (the
one between the housing and the block).

I don't know nuthin about no gas in oil - sounds bad.

mike