10-40 is the recommended in the manual. I once used a 20-50 and the bike became to noisy so went back after 1000 miles. I now run 15-40 and I wouldn't recommend anything thicker than that on a water cooled engine.
I run 15w40 dino oil, as far as what brand... well, plenty of opinions/arguments/discussions out there, just watch out as some people get VERY angry.
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I agree dont run 20 50 as for i was also noticing the louder trans noise so back to a quiet running 10 40. Well not to quiet these bikes are just a little bit noisy with valves ticking away like crazy. But all good as this is what this engine sounds likel.
This subject came up on here a while back and there were some tense moments! LOL. I just did my first change myself after the 600 mile suggestion in the manual. What I took from the majority of the entries is that you shouldn't use synthetic for the first couple changes since it sticks more and won't drain out the metal shavings as much? So...I went with Valvoline 10-W-40 as in the manual and used the 2002 Infinity Filter for $6.00 from Napa as reccomended on here. Fit well, tons cheaper than the OEM and others. My dealer said I have to come in as he has the tool you need to get old filter off. I bought one at Napa for $5.00. Have to make sure the oil is CA listed on the bottle for the wet clutch. Not all car oils have that, the Valvoline I used said ATV on the label and was $5.00 per quart.
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The upper range in the engine spec book for 10W-40 is 104° F (40 C). There are times here in KC where that is exceeded. Not often but it happens. So I run 20W-50 Mobil 1 V-twin oil during the summer months and 10w-40 Mobil V-twin during the rest of the year. That's a quandry because there are times in winter where the bottom of 10w-40 isn't low enough. My bike is inside at nights but sits outside all day. Usually the temperatures raise above the lower limit before I leave work.
I haven't noticed any loud noises with the 20 w, in fact it's usually quiter. but it's extreme cases where the 10-40 is exceeded and the vast majority of bikers aren't going to be out riding in that junk where some of us just have the commute home.
This is just what I do. Nobody else has to do it, just because it's in the manual doesn't mean you need to do it. Heck, half the people have problem reading that thing for Octane ratings let alone a temp chart for oil.