Oilers And Tankers

What's the difference between these two types of ships in real life?

oiler refuels ships. A tanker just hauls petroleum.

Then why in the USN sub campaign, around the 3rd/4th mission, does the storyline mention the importance of taking out oilers but not tankers? This is the mission before you're told to take out everything - even merchants.

Kind of contradictory.

Also, there's another contradiction in the that 4th or so mission, where the merchants are to be sunk. When the mission begins, it says in the control panel's message text that the enemy has introduced an as of yet unheard of weapon called hedgehogs. There were hedgehogs in the campaign's 1st mission. Ooops. sleepy:

If you take out the oilers, the enemy warships are screwed because they won't have enough fuel to continue, or maybe even get home! Oilers are "always" more important than tankers

Doesn't a tanker transport crude oil and not petroleum? Everytime you hear about a tanker accident on the news they always mention leaking oil and the effects on wild life.

A tanker with refined petroleum would be one big explosion waiting to happen would it not?

Some tankers transported crude, others refined petroleum like aviation gas. BIG bangs.