Hot SLK

How long should an SLK engine stay hot? Specifically, a '98 SLK 230 (which I bought two days ago).

I spent 4-5 hours driving around in local traffic, normal stop and go. I used the ac a little bit, but mostly not. The outside temperature was probably about 80 F when I started, and 67 F when I got home at

Checked the car just now. The engine and radiator cap are still hot to the touch - FIVE hours later! Is this normal??

BTW - if it matters - the oil and filter were changed when I bought the car, i.e., two days ago, using synthetic oil.

Thanks as always.

--Jonathan