Trolling around over at Stangnet.com I came across some of the posts by Cobra'03 - here, let me give some links. He knows what he's talking about, and its all a good read.
On additives
On Gear lubes
Synthetics in General
On fuel injector cleaners
Syntec is crap
On weight of oil
more lube facts
Change intervals
Why SVT says no synthetic and why they want you to use 5W20