Big - there is a whole lot of discussion on this here on the forums. Basically, our engine is a flat tappett, strong spring engine that requires more protection in the forms of Zinc and Phorphorous in the oils we use. But by todays standards those two items lead to premature CAT death and thus oils today have much lower rating for those items to help with the longevity of your CAT (not to mention the valvetrain has changed to where the parts don't need the same protection).
So really you want to run an oil with a high ZDDP% (Zinc and Phosphorous) to help protect our "antique" engines (yes they are considered antique). Minimum you would want would be something running a ZDDP% of about 1100ppm (talks on here an other forums say that you should truly run about 1400-1600ppm oils for full protection) so what we find is we have to purchase expensive oils that have these higher ratings or HDEO (diesel) oils. Unfortunately, most mfgr's are not very forthcoming with this information so you have to make an educated guess. I think until recently most Castrol GTX has been pretty good but I want to say that it has dropped in ZDDP% in the last few years.
For A LOT more information check out the site... BobIsTheOilGuy.com and go into their VOA (Virgin Oil Analysis) sections and look around. I have found that the ones with the highest ratings, that can pretty much be picked up anywhere, are Rotella T5/T6, Delo 400LE, and a few others. Again, unfortunately for us with the changes that have been moving towards lower ZDDP% oils, stuff for our engines is getting hard to find and much more expensive.
Note: here is a whole thread on the 5W30 where they were not very impressed with the numbers, also found a few other muscle car forums that were showing the same 800ish ZDDP results of the DEFY. Personally, think I am going to stick with Rotella, Delvac, or DELO.
