Personally, I have usually used an oil based primers especially for cedar and pines. It has always worked.
However, I'm studying for my license and it saids " if the wood has a high oil content, sealers must be used to keep it from bleeding."
I don't know what they are talking about. I've always used an oil based primers, not a sealer.
Are they using the word sealer and primer interchangeably since primers are designed to seal?

I'm not familiar with whatever you're studying (and you didn't specify where you are or what it is...) but, yeah - it's talking about stain blocking primers, the best of which are oil based products.

I stand to be corrected / further educated.

 

Priming is a universal word. Sealing is indeed a form of priming. Oil is still superior in this regard.

 

Primers are for adhesion, sealers are for sealing.

Can some primers acts as a sealer, yes. Can some sealers prime the surface, yes.

 

Tannin bleed is an issue with western red cedar And some other wood types. They make special sealers for that. Even then???