Anytime you buy a USED car or bike, change the oil immediately, because you never know. I bought a Vulcan in 2004 ('03 model with 6K on it), and let the oil go thinking, they are a dealer, they do what's needed *WRONG*!! The clutch started getting real chattery and grabby when hot, and just didn't feel or sound right, the bottom end knocked like hell. SO, after about 1000 miles, I decide to change the BLACK oil. Well, the bike takes about 3.7 quarts, and I only got 2.5 quarts out of it!! Yep, the oil was low, and the low end of the bike made noise. Well, that all stopped with the oil change... If a DEALER can't get it right, then don't bet on a private owner doing it right either... Even with new 20W50, when the bike got real hot, the clutch was sloppy, not as bad as before though...
Anyhow, I traded in the V800 Classic on the C50T, when it got hot, the clutch felt like the Vulcan did, so after the break in, I went with Synthetic oil, and the clutch is smooth ALL The time now. I used 20W50 JASO certified Synthetic, as the user manual said you can use that in certain temperatures, and I will never ride in the winter.
When I had the Vulcan, I used regular 20W50 motor oil, and I put over 8K on the bike with no problems. There is no real mechanical difference between a car 4-stroke and MC 4-stroke in how they WORK, except the RPM range is higher with the MC, and other things, thus more friction, breaks down oil faster, which is why Synthetic is a good idea.
NEVER use friction modifiers and additives with a WET clutch, it will/can cause the clutch to slip on hills and hard throttle...
People will argue with me about oil, but I have owned SEVERAL bikes, all Japanese, and I even had bikes with over 98K miles on them. I always used regular 20W50 motor oil them, all the time, and I have NEVER EVER had a problem, or mechanical failure. I Sold the bikes, and no one ever brought it back with a bad engine, or complained later about it. Just as long as you DO NOT use the Energy Conserving oil, even the synthetic type. Stick with 10W40 in colder climates (for starting) and I always have used 20W50 in my region, for ALL my bikes.