If you just finished ARX on both and have no leaks, then your seals should be fine. If you are not consuming or leaking any oil on either vehicle now while using dino oil, you should be fine using synthetic, esp. on the newer one. My question is, why do you want to use it? Engines will last long long time with good maintenance using regular, good quality dino oils at 3-5000 mile oil changes. If your plan is to extend your OCI significantly, then there may be some benefit to using synthetic oil and a premium oil filter.