I am running Pennzoil Platinum 5w30 for this oil change. In Louisiana the temps this summer will push 100 but I haven't noticed any difference from 5w20 after 1000 miles. Don't know about GF-6 but with GF-5 5w30 would shear down to 20 grade or close anyway. Depends on sale rather I run 5w20 or 5w30 oil. Now synthetic is quieter that syn blend or conventional. This is my observation in 13k that I put on and it has 16k on right now. I change every 6 months but am going to try to go a year as manual states. Lots of short trips.