Thanks, Doc! I knew I’d have to wait a bit for several fillups to see if my mileage had actually changed – my driving’s pretty regular, though, so that’s why I wondered. Other than the oil change, there was nothing different I did before that fillup.
Having once been a proud and happy owner of a diesel Rabbit, I was used to changing oil weight according to the seasons. Didn’t realize some of the other stuff you mention – thanks for that (0W30 … whoa!). The funny thing about this whole mixup is that I was buying synthetic – they had a good deal on it, and I just spaced out and grabbed one weight, forgetting that the two cars took different weights…
Thanks to all of you for your responses!