Just bought a silverado 1500 with the 5.3L and the 4l60e auto trans. Has 150k on the clock. There was an oil change sticker in the window so I called the place to see what they had put in the truck when they did an oil change for the previous owner and they said regular 5w30 so to me that means conventional. According to the sticker it's time to change it again and I checked the level and it is where it's suppose to be so it doesn't seem like it uses any oil. What oil should I go with for it? I want to go full synthetic. Just trying to determine HM or not. And what brand