testing an m6

Hi guys, I think I talked about my m6 here before, but any hoo, story goes I found a buyer but his test role came out blank. After some testing I discovered it was fine with light films like fomapan and rolie, which I was using, but would start slipping at frame 5-20 on heavy films. I've been tinkering with it for some time now and it seems to shoot perfect. (HOW DO I TEST IT FOR SHURE THIS TIME). I might go eBay rout so don't want any hassle, I have put it down to the rewind switch sticking, and after some oil and uses it must have loosened up. but tell me, whats the full testing methods for these cameras. I have a role of tri-x400 which is the heaviest film I have. ''is that heavy enough?''

My Leica Ms are taking anything from thin Polypan F to thick Kodak 50D.
This is what normal people will do:
Test any Leica camera with any film. If it doesn't works with any film, bring it for service. M6, M2, M4, M5 doesn't matter. It has to be CLA'd by person who knows what he, she is doing. Or list it with defect described.

But you chose to dump some oil without any clue and now asking how to test camera which was required CLA. How do you test something you just made worst?
If you don't want any hassle on ebay, list it with defect description. At least. Most correct description after you dumped oil into it is "I f#ched it up", but ebay will not allow this.

LOLO!!, dude, go take a cold shower. ''seriously it works''

il be more specific, What is the best test film, aka the strongest one there is? to test my ''was'' broken, now 10 roles in and rolling like butter ''AFTER MY REPAIR WORK'' Leica M-6. that I'm paranoid about because of the embarrassment of it failing for the buyer, after me shooting away 5-6 roles of Fomapan no prob before.

Good luck with the camera, but I hope you'll warn any potential purchaser that you added oil to it, because that is a great way to cause serious damage if the excess oil isn't quickly removed through the process of a Clean, Lubricate and Adjust ("CLA") procedure performed by a competent, trained technician equipped with service manuals and the necessary specialized tools and equipment.