[F30ed]

Filter gasket fails on all of them. Check head cover gasket too, it must be leaking at passenger side at cylinders 5-6.
You are lucky in the way that oil leaked to coolant, not vise versa. Othervise you would be shopping for new engine. Ohhh... check/change alternator belt now! if oil got on pulleys it may slip off and get sucked into the engine block thru main seal - that will kill the engine for sure.
Replace thermostat, pump, oiled sensor and you should be good I guess.

P.S. if your so good mechanic did not see a major oil leak - look for another mechanic.