I do know with the front seal that there are MANY versions and materials. The last one that I received from the Classic Center was a light brown color and had a three letters that indicated it was molded with a high temperature and chemically resistant Fluoroelastomer. Viton is DuPont's trade name for that and most recognize that. Probably about as good as you are going to get and superior to the original Buna-N or (Nitrile) material.
There are also a couple of tricks like replacing the crankshaft sleeve and using sealant on the end of that where it mates with the crank gear. These can help ensure a leak free installation. Others that are experienced with this can elaborate.