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You already have a synthetic oil when you use Mobil 1 why do you need to add anything to it. As for adding something that is supposed to help worn out engines why would you want to plug up the small holes and spaces in a perfectly good engine.
According to the service manual,Mobil 1 is the recommended oil to use.
The service manual actually says to use an oil meeting GM Standard 4718M. Mobil 1 is factory fill. No where in my 02's owner's manual does it say that Mobil 1 is recommended. All it says is that "Your vehicle's engine is filled at the factory with a Mobil 1 synthetic oil, which meets all requirements for your vehicle." This doesn't imply that this is only oil on the market that meets the GM Standard 4718M requirements.
As for oil additives, AMSOIL, along with just about every major oil company, says not to use them.
Regular oil and filter changes do wonders by themselves.My golf buddie has 97 Sonoma he bought new,change oil & filter every 3 to 4 thousand with regular Penzoil.He has OVER 550,000 on the original unmolested motor and still going strong,No additives involved.