Bill, I can't speak for all synthetic oils but actually most of them can readily be mixed with petroleum based oils with no problem.I am an Amsoil dealer and our oil does mix with any other oil with no problem.If you're not familiar with Amsoil,all of our products are very high quality, 100% synthetic lubricants.I have been using Amsoil for a couple years now and had to switch several motors from regular to Amsoil and had excellent results-less smoke,less carbon,lower engine temps(I've used it in 5 snowmobiles and 2 outboards).Amsoil offers many other lubricants also,such as lower unit lube,grease,gear lube,motor oil(4 stroke)and much more.If you have any questions just E-mail me.