Over the weekend I traveled home and back: about 140mi each way. About half way there the engine light came on, and, realizing it had been about 4500 miles since my last oil change, I added oil to the engine after arriving at my destination and confirming a low oil level. The car ran fine and the engine light did not turn on again until about half way back the other way. Unlike the first time, about 20 minutes after the engine light came on I noticed a lack of power at higher rpms and while driving uphill.
The next day I took it in to my mechanic to get a diagnosis. There were many, including a timing belt issue and an issue with the variable cam. He suggested starting with the cheapest solution first, and changed the oil and oil filter. When I got the car back I asked him if there was anything in the oil he drained: there wasn't, but he mentioned something about it smoking slightly from the exhaust. On the trip home I noticed smoke from the exhaust (it was not slight) and the same lack of power. I thought the smoke had a blue tint, but this morning it just looked grey. The mechanic told me to drive it for a week and come back if it continued to smoke.
There is no smoke from the engine compartment, and there was no smoke before the oil change. There are no dash lights on either. Also, the idle (and at all other rps) is smooth and it starts right up. Why would it have start smoking after an oil change and what would cause the lack of power? (not accompanied by smoke the first time around) Should I take it to a Subaru dealer, or to someone else?
Additional info: the car has 98500 miles, and the timing belt has not been changed.