1976 Mercedes Benz 450sl Car is mixing gas with oil

Depending on how long this has been going on, you may have already spun the bearings due to poor lubrication. A total rebuild would be recommended if the mileage is rather high, 1ooK plus miles. Cost could range from $ 4,000-$5,000.

Do you have oil getting into your fuel (lots of blue smoke out the tailpipe)?
Or do you have fuel getting into your oil (oil level goes up, the oil gets thin, and you spin bearing and the pistons chew out the sides of the cylinders)? Either problem is more than a "not very car technical person" would be able to fix, but knowing which would help you when you take it in to have someone fix it.

I do have to break it to you that a quality engine rebuild on the M117.985 engine - the one in your car - isn't $4-5K. It's more like $8-10K.