Is wintergreen oil in toothpaste effective?

Not really: Wintergreen oil is sort of caustic and bad for your gums. It is a flavoring but doesn't do anything except impart the wintergreen flavor. Best to stick with a known brand of toothpaste and without all those extra things that don't change or improve your oral health.

Possibly: Aromatic oils such as wintergreen have a variety of reputed beneficail effects. I am sure an internet search will yield a plethora of information on the subject.

Possibly.: Wintergreen can be used in toothpaste in the proper proportion, but as far as effectiveness, you didn't say what it could be that you're looking for effectiveness with!