Have been running Mobil 1 0w20 in my 2018 Subaru Forester for 40,000 miles (since new) and have averaged (according to the car MPG display) 39.7 MPG for the last 5,000 miles. Decided to fill with Amazon Basics in 5w20 full synthetic during the hot Pennsylvania summer months. Have put 2,500 miles on the Amazon Basics 5w20 and my mileage has dropped to 37.7 - a drop of 2 MPG. Driving habits still the same and I'm the only driver. One difference is that I've been using my AC more because of higher temps this time of year. Is this a common phenomenon when changing grades as I have done? No other changes have been noticed, e.g., startup noise, etc. My intentions are to go back to the Mobil 1 0w20 next change which should be September when the temps average a bit lower. Your thoughts?