Viton or Buna-N, and how to tell without guessing
The seal is the cheapest part of the element and the most common reason one fails early. Temperature is most of the answer.
Seal material is the second most common thing to get wrong on a replacement element, after the length. It is also the cheapest thing to get right.
Buna-N is the default and it is fine to about 180F. Above that, and in anything with a phosphate ester fluid, you want Viton. If the reservoir is hot enough that you would not hold your hand on it, ask us.