the key factor these days is the zinc content. most oil manufacturers have removed it do to its ability to hurt oxygen sensors when the vehicle is running improperly. i use diesel oil in all my flat tappet builds instead of the recommended (zinc additive) so i recommend rotella(2nd highest zinc content) or chevron delo 400 (highest zinc content)... as for synthetic vs non synthetics... of course the synthetic will show better viscosity levels and more consistently but it should be ran from day one... as the engine ages the high viscosity levels have been rumored to hurt it because the oil does not break down like a conventional oil once the engine has wear. but the other side of the argument is if you use synthetic from day 1 it wont wear like it would with conventional oil.... i dont use it... i personally had an engine spin a rod bearing under normal driving conditions 100 miles after switching to mobile full sythetic and for that reason i will never touch it again. also another thing to note with the diesel oil it will have the anti-foaming agents in it because of the turbochargers and there 40-50,000 rpm impeller speeds so if you have a high rpm small or big block this is a nice feature. and for the detergent vs. non detergent....i was a heavy line automotive mechanic for the last 7-8 years and it has been my personal experience the detergent based oil is probably good but when you get a vacuum leak or it over heats a bit or it is neglected here and there... the pariffins in the detergent turn in to a wax build up when mixed with air...and i am sure some one here has pulled the valve covers on a small block chevy that has seen a clay wax build up on the rocker arms from the pennzoil and bad valve seals on a neglected engine... and there is no denying it was pennzoil or quaker state because it has a very distinct smell...from a year of working at a speed lube while in college...lol i am sure this may not be as technical as it could be but these are the reasons why i do things and the ways they were explained to me. hope this helps you make your choice.
I've ran conventional 10W30 for the 12 or 13 years I've owned it, but it will be filled with 10W40 synthetic when the motor goes back in this spring.
