IMO, it's best to change your oil, esp. on a Toyota/Lexus. If you don't know how to, good idea to learn (it's a simple process):

1. You want to ensure the correct oil is going in and at the right level. There is a good chance Toyota/Lexus and the other shops will overfill.
2. Most importantly, most Toyota/Lexus vehicles use an oil filter housing canister (compared to a traditional spin-on oil filter) that you do not want to overtighten (the torque specs on this is about 18 foot/lbs., which is not much) and there is a good chance they will overtighten. If you have a plastic housing canister, overtightening this will cause it to break (good idea to replace the plastic with an OEM metal housing canister).
3. You will save the most money and time by doing it yourself.

I also have a GTR that I refuse to take to the dealer because I want to ensure any maintenance is done and done correctly. Just save your receipt and keep a record of what you have done for warranty purposes. In regards to warranty, I don't believe in it because you have to move mountains for the dealerships to honor the warranty and the burden of proof will always be on you regardless of who does the maintenance.