Viton Oil O-rings
 

I posted this question to the "rotary performance" section but got no replies, so I'm going to post it here too.

I found a large variety of Viton O-rings at McMaster-Carr, but Atkins calls they're O-rings "ARE 15". Mcmaster O-rings are 75A Shore. What's the difference?

Anybody have any advice, knowledge or experience with this? Are Mazdacomps Oil Control O-rings the same as the Atkins O-rings?

Thanks,
Eric.