Been using Castrol synthetic since the late 90's. No problems. I use it in several high milage Toyotas and even an old Briggs L head generator built in 1986. Keep in mind that most synthetic today is highly processed dino oil. Shell and Mobil fought over the terminology
in court. Shell, which developed the process of highly processing mineral oil until it performs as well as "true synthetics" won. Google it.
Shortly thereafter, Mobile switched to making Mobile 1 the same way. There may be a grade or two of Mobile 1 which is still true synthetic, perhaps the racing oil. Bottom line, the new synthetic isn't going to affect seals any differently than "regular" oil. "Bob is the Oil Guy.com is a site looking at.