I just got an 03 g35 coupe 6speed. What brand of oil and what ratio is best for this car?

There are literally hundreds of threads covering this topic in the Engine/Drivetrain section. As a new member one of your responsibilities is to do you own leg work finding answers to commonly asked questions such as this one. Failure to use the search function can/will lead to infractions and we don't want to do that to you. Please read and follow the forum rules for a long and prosperous stay with us.

Congrats on your new car and welcome to the family!!