Oil Question on a 94

I have a 94 Gmc Sierra, and i am going to be doing the full SeaFoam Treatment , and want to know what would be a good oil to use afterwords. I hear synthetic is the best and that is what i am running in it currently *Mobile 1 Full Synthetic * and we'll i was told Royal Purpal is one of hte best out and my consern with that is i have around 129,000miles on my truck right now would i end up ruining anything? If not besides royal purpal what would be another good oil to tryout.

Thanks.