synthetic HDEOs like rotella 5w40 are slightly higher additized than the conventional 15w40 HDEOs so they are a step up in protection, cleaning, cold flow, oil change intervals etc. As far as damage from a thick oil on cold starts, most wear occurs at startup and the thicker your oil at startup the more wear occurs. Now the question is will you see the difference over say 100k miles from that wear, I'm not sure but if your are hearing valvetrain noises like that it is from lack of oil flow so use your best judgement..