Yours would have a dipstick, I do, and my car is newer, '67 250 SL. You need to remove the dipstick, suck the existing oil out and replace with your recommended motor oil checking the dipstick to be sure you do not overfill. Overfilling will result in, I think, a richer mixture, but I'm not sure. I do know that overfilling changes the mixture.
The dipstick can be hard to turn by hand and you may need a tool, long pliers, etc. It is a bit clumsy.