What oil you use depends on the mileage of the motor. You didn't say whether it was a 6 or and 8. V8's can handle a thicker oil, say 20w-50, and don't waste your money on synthetic stuff.
Castol 15w-40 for winter, Pentrite 20w-60 for summer. 6's could probably take a thinner oil (OHC and all that stuff)
The most important point is to change it at 5-7,000k, no matter what. Synthetic oils are a con, because the additives still decompose after a certain time, so your detergents,viscosity improvers, etc, etc, still break down after time, so a cheaper oil changed more often is better for your engine and the ole hip pocket (or purse, if you are that way inclined)
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