As 555 said is the way to do it, no problems, just let the engine idle and all will be well.
As for serviceing goes at at dealer. The European Block Excemption ruling states that it a car is serviced and repaired by qualified personell using parts of similar quality, that warranty is valid, and that's the law, for Subaru, and everyone else.
How many miles on the car since it was serviced, it sounds like the oil is not up to the job.
The tappet adjustment is a big job alright as the engine has to come out, but if they are noisey they should be done.Checking the clearance can be done without takeing the engine out. The noise is not caused by the gap getting larger, but getting smaller and the flank of the cam lobe strikeing the bucket. next step is the vavle (usually exhast first) not closeing completely.The tappet usually does not wear, it's the valve sitting into the seat more, thus reduceing the valve clearance.
John