Okay so driving to work yesterday my 98 SLK230 was overheating while on the highway, but if I pulled over and stopped it would quickly go down in temperature. It was my first time taking the car on the highway and has never overheated on me before. At lunch I let the car run at idle for half an hour and it didn't overheat at all. The usual 20 minute drive home took me an hour last night because the car kept overheating, and eventually oil was spewed from the passenger side of the engine and the whole underside is soaked. It did get into the red a handful of times, but I would shut it off immediately. So I'm hoping it isn't a head gasket because if so the car will quickly become a parts car and I will be selling parts haha, but are there any other causes of the leak on that side?