Hi and thank you for your question.
Motul is a quality product. It is surprising to hear you encountered problems with using it. My first question to you is the Motul you are using synthetic oil? The Yamalube 15W50 is a full ester based synthetic oil. Ester based synthetics are the highest rated oils used in motorcycles for engine protection. If you have gone from that fully syntethic to a Motul petroleum oil and are having shifting problems, the easiest solution is to go back to the Yamalube. Stick with what works. Now, if availability is the issue, find a fully ester based synthetic motorcycle oil and give that a try. Full ester synthetics are going to be the most expensive of all synthetics. All synthetics are not the same. There is a reason some synthetics are $5 a quart and some are $20 a quart. The main reason for the price difference is the type of synthetic you are buying. Overly simplified, there are oils that are called synthetics that are petroleum based with synthetic additive packages. Those are usually the $5 synthetics. Then you get into cracked carbons that are similar to highly refined petroleum oils with synthetic additive packages. They usually fall in the $10 price range give or take a couple of bucks. Then you have full ester synthetics which are the best protecting synthetic for automotive/motorcycle applications. They are the oils that would typically run in the $15~$20 range.
To address your 2nd question, about it possibly being something else. Yes, it could be BUT it is highly unlikely. You have tied the problem to a specific event (oil change) and that is always the best place to start when diagnosing problems such as this. I would try Yamalube again and see if the problem goes away. Right now that appears to be the only thing that has changed and now you have a problem.
thanks for reply
ill go back to yamalube and see if that works
the only other thing ive changed was to put a new tyer on the back , i followed the manual for chain adjustment between 20 to 30 mill slack
could that have any thing to do with it?