I've got a 1976 450SLC and the oil filter is fairly easy to find if you are underneath the engine. It's a canister oil filter and is aluminum in color with a big bolt on the very bottom of it. Once you loosten the bolt and drop the holder, you have to replace the paper element (this is the oil filter itself) inside. There is usually a replacement oil plug gasket (a copper washer) that comes with the new filter.
If I recall correctly, it takes 8.5 quarts of oil if you are changing the filter at the same time as the oil. So you'll want to make sure you've got a big oil drip pan. Good luck.