All engines make noise. With a motorcycle you sit on top (more or less) of the engine so you hear a lot of the noise. If you had loud exhaust on before, the sound of that probably covered up the engine noise.
wear a helmet, and then take the helmet off and ride, sounds like the bike is coming apart... The louder Muffler covered this up.. things is you will have to re learn what is normal and what you need to be worried about!
Unless you are in a really warm-hot climate, I would stick with 10-40 because it'll reach the top end quicker on cold starts. As for the clatters and ticking, if not harsh, would believe they are normal.
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