As far as base stocks go, both the Amsoil Sig Series and the RP XPR use a healthy dose of PAOs in their base stocks, so unless it is a radically different additive pack, or more POEs (esters) in the (base stock) blend, I don't see how the XPR would be any better than the Amsoil SS as far as standing up to meth and E85 goes.
Unless it is some other additive completely which RP adds into their racing oils to be able to specifically stand up to those caustic fuels??
Did RP tell you one way or another that their XPR is purposely designed to do just that?
Yeah, they did. For performance (street) applications, Amsoil actually recommends their Zrod oil, not the SS. For racing apps, they recommend their Dominator oil, but that oil is not meant for any type of extended drain, race it, and change it.
With Royal Purples XPR, it can actually be used for extended service (3k miles) in a street car. I've just now switched over to it, I'm gonna run it on the street for a bit, then send it to Blackstone and see what they say. The XPR also uses alot more POE's.
"Using XPR with Exotic Fuels
Royal Purple’s lubricants can be used with exotic fuels. For the best protection, Royal Purple has formulated its XPR 5W-20 and XPR 10W-40 specifically with this in mind. These two oils are formulated to combat fuel emulsification to hold up even better than Royal Purple’s other racing oils in alcohol and methanol applications."
"Demulsification--Separate rapidly from water as well as exotic fuels, such as methanol, which are typically detrimental to lubricants."