I am thinking of switching to a synthetic oil for my 68 pontiac firebird 400. Up until this point i had never really considered synthetic as the big 6.6 liter at 400hp is not all that hard of a working engine (redline is right at or slightly above 5k). it does however run a little bit dirty compared to never designs and pontiac v8s are known for running a little hot. the car is driven approximately 2000 miles per year and sits for 8 months with only periodic start-ups. I am wondering if it is time to upgrade. the car has hydrolic lifters and basic stamped steel rockers. I am wondering if there is any danger is changing to synthetic after all these years of conventional. can conventional and synthetic be mixed? any thoughts? is mobile 1 as good as say...royal purple? I run 10w30 all year in her.