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Join Date: Nov 2007
Location: Chandler, Arizona
15w40 would be ok unless it is really hot. The older an engine gets, the more wear it has, and the looser the clearances are, such as the crank bearings and the con rod big end bearings. You need a thicker oil for a high mileage engine to help fill in those looser clearances. But weight of the oil did not cause this problem. Neither did overfilling, especially if you changed the filter. I've always just poured a gallon in mine, and it comes out perfect, with just a tiny bubble at the top of the sight glass. All VN750 engines are built exactly the same. Some (probably most) have been modified to some extent, but nothing that would change oil capacity. This is something else.
I remember when I had a Kawasaki EX500, and belonged to the EX500 forum. There were a couple of cases where the bike was dropped, then picked up, started, and ridden away. Shortly thereafter the engines seized up from lack of oil pressure. Apparently while the bike was on it's side, the oil pump lost prime, and did not start pumping again right away. You didn't say anything about dropping your bike, but you did leave the drain plug out for a week. Did the OIL light go out as soon as you started it after changing the oil? Also, the Vulcan 750 has hydraulic lifters. Maybe yours are leaking. But I've let mine sit for as much as 2 weeks (with oil in it) and never had a problem. That sounds more like a clanging sound rather that a bearing knock or a cam chain rattle. You also said it did not make a noise at low RPM. Cam chains and bad bearings will both make noise at idle.
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike