I've used Rotella 15W-40 for many years in my XS650D. When Shell cut the ZDDP content I started adding a bit of ZDD-Plus to keep the cam and rockers happy. Just guessing, but Rotella might not have been compatible with the Mobil1 still on your clutch plates. Shell has started selling a semi-synthetic Rotella 15W-40. I don't use it, it's not rated JASO MA.