Years back I had a Camaro hot-rod that saw only summer usage. I learned my lesson with car covers after having to repaint. They won't keep dirt off the car, and the cover can move, scratching the paint. The next years I just let the car get dirty and then carefully washed it come spring. The paint was as perfect as in the fall when the car went "away."
Change the oil just before putting it away, and change it again in the spring. I also used a battery-tender to keep the battery in good shape. If possible, elevate the car off it's suspension to keep from "flat-spotting" bushings, etc.
As to the FSS, I use it. I change the oil/filter every time the FSS counts down 5000km's. No way would I let the oil sit in there for 15K plus. Then, I do the A and B services when the FSS gets to zero, and I alternate A and B. I do all my own maintenance work, and you can too on the SLK. The only jobs I farm out are changing the coolant and tranny fluid. The coolant change is easy, but my local dealer only charges $40 to do it. The tranny fluid on the 722.6 five speed automatic is a tougher job, and since it needs doing only once every 100K-km's, I have them do that too.
1998 C230 "Black Betty"