just finished doing that exact process thanks!I would use and have used varsol. Its oil based and everyone knows varsol tanks clean grease and oil. Add in 400ml of varsol or paint thinner(same thing but cheaper), run quad on jack stands for few minutes to clean diff., then dump. Add in new oil, a little but less than normal, run for few more minutes then dump that. That will get out all of cleaner. Add new oil to full and have fun...That worked for mine..