If the engine is started from the first oil change on schaeffers, it will not only keep it clean but will not allow any varnish build up. I have one guy that has used the 15w40 in his car since first oil change. Had a timing cover leak underwarr. So after pulling the timing cover, oil pan and valve cover the dealer called him and asked what he was using since they have never seen a 36,000 mile motor look like that did, which they stated was like it just came from the factory.
I wish I had gotten pics of that one. To answer your question, yes, it will keep the engine as clean as any "full" synth.