I went to the local dealer at about 6K miles to get the free 5K service. The invoice said synthetic service, so it should be the 0W-20. Now the average mileage since the last fill-up is lower by at least 3-4 MPG. My driving habits haven't changed. I inflated the tires to above 40 PSI. I looked at the oil. It is medium amber colored. (I forgot to look at it before the oil change. ) What does the 0W-20 synthetic typically look like after an oil change? I'm wondering if they really used the synthetic, even though the oil cap clearly says 0w-20. I'll wait until the next fill-up to see what the true mileage is before I question the dealer.