M Visa to Z Visa

I am currently a M Type holder multiple entry's 60 day limit. I work for an American company and I am being paid out of the American branch. However I am in China helping out the China branch but yet still being paid out of the American side. I am a business traveler that comes and goes a lot. usally not staying for more than 25 days at a time. However I have been told that I need to get a Z type because I plan on being in China for more than 90 days in a calendar year. (even though I am always out before my M type 60 days expires.)
I plan on coming back and forth and have a total of 170 to 180 days in China. But don't want to give up my M type. What should I do?