How do your plugs look?

If it is exhaust carbon then your bike is running rich. I am not familiar with your bike's fuel system but look into it. I do not see how an oil change could cause this problem. It could also be brake dust. What side is your rear rotor on?

I agree with Mike (Rex), if you are seeing excessive carbon in your exhaust, your probably running rich, or it could be bad plug wires, or the use of an incorrect spark plug for the engine. One thing that could cause your bike to run rich is a dirty or clogged air filter. 

But don't forget there is some carbon in all exhaust, and if you have something in the path of the exhaust, like a turn signal, you are going to get a carbon film on it.

Does you spark plug look like this...


If it does, I would check to make sure you are using the recommended plugs, your spark plug wires are in good condidtion and you air filter is clean.  If those are ok, you should change your air air/fuel mixture to run a little leaner.

My plug doesn't look as bad as the one in the pic but it's headed that way.
As far as the location of the turn signal, it's offset to the left by abouy 2 inches.

Sounds like the problem i have with my Zuki. It fouls the front plug. I went one range hotter on spark plugs, seems to help. Good luck with that. Don't know about your C50, but the tank needs to come off my Marauder to get to them.

Make sure your oil is not over full and allowing oil to enter the intake and get burned.
Like I tell people all the time at work. "you didn't have a problem, you worked on it and now you have a problem, where should you start looking first?"

Just noticed your problem. Left side is shaft drive side and the turn signal is just below drive. Check for drive oil leak, Transmissin or differential. Slight leak of the engine or dif case oil could be the problem. Rear cylinder will run a slight richer to help cool but if from exhaust would be all over and could smell oil smoke when shut off after hot run from pipes if turn signal is wet.

Yeah ~ and a 'gas carbon' is going to be 'drier' than 'oil film'... unless you have cats. I've seen engines go through oil like a sieve and still have a 'dry pipe'...