Tankers Or Oilers?

I'm on the 3rd mission in subs, I've been sunk about 8 times tryin to get through this one mission, I was doin pretty good til I updated to gold,LOL. But I'm havin troubles distinguishing the oilers from the tankers to know which ones to sink. Can anyone tell me where to find a screenshot of each or tell me the difference? Thank you very much!!

Sink 'em all!! >:(

I'm redoing the missions myself but don't they have different designations thorugh the periscope?

The Oilers are all O designation: O1, O2, etc.

Oilers have 3-4 masts (can't remember exactly), and tankers have no masts.

Thanks for the info guys!! Lookin forward to fightin with or against you guys when the multiplayer finally comes out!!