My TCH has more than 7,000 miles now (they grow up so fast). At my first oil change, I switched to Mobil 1 synthetic oil.
I just finished a spreadsheet to compare mileage before vs. after synthetic, and will continue to compare as I add new tanks.
The MPG gains with synthetic oil seem to be impressive. I discarded the first two tanks of data after the oil change, because the dealer had lowered my tire pressures to 30 PSI or below, when I had been running 40+ PSI.
When I compare my tanks BEFORE switching oil to my tanks AFTER switching, there is a 8.15 percent gain in MPG using synthetic oil. Will keep you posted on whether this significant gain may continue over time. So far, I'm liking it.
Regular oil: 36.2 MPG average; synthetic oil: 39.1 MPG average. YMMV.