Cooking mushrooms in anything other than butter is a sin!
However it's worth remembering that the constituents of some vegetable oils might have been changed (possibly resulting in them only being able to achieve a lower temperature without going up in smoke) because a great deal of the vegetable oil used in this country normally comes from Ukraine.
Actually, I quite like them quickly seared off and then served, still retaining a little bite, maybe a little herb like lemon thyme added - and a turn of black pepper......
Agree with above butter is best. If uoure seeing a lot of black oil or other extract from mushrooms that does suggest they're a little psst their prime. Try to cook them before they get there and reheat as needed, they will keep 2 or 3 extra days cooked.