I had a 1993 S10 Blazer with about 89k on the clock at the time.. I put mobil 1 in and leaked like a sive It actually blew the remote Oil Filter seal..Synthetic will cause leaks if you already have a leak pending..... Trust me... 0 oil pressure is not good!
I have Castrol synthetic 10W30 in my 4 banger and oil pressure jumps over 100psi when the engine is cold and hangs between 60 and 80 when it's hot.
I will run a flush detergent with cheap oil for 10 minutes (while the engine is in the van) and switch to full synthetic 10w50 for my 5.0L Mustang. Once the 351 is done, it will run with full synthetic 30w or 20w50. Because viscosity (sp?) breakdown isn't too good for the engine
Also when you take apart an engine that has ran on full synthetic, it's much cleaner and not as worn out. A racer near my place used to rebuild his engine every season and since he started using full synthetic, he doesn't need to rebuild the engine as often That is enough proof for me to use synthetic. Since then EVERY car I've owned ran on that stuff except for my old 3.8L Cougar which had some 50w racing oil LMAO and still couldn't get over 25psi oil pressure (0 with 10w30 oil) lol