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First off, I’ve tried searching with no success. I’ve been lurking in the 40 section for quite some time trying to learn what I can while I shop for the 40 that checks most of the boxes. I’ve learned quite a bit but there’s a couple things that still leave me stumped. 1) Rear fender lights...

Is there any reason NOT to use Royal Purple Duralec SAE 15W-40 in a 1HDT?