"Gunk" makes an excellent engine flush add to oil allow engine to idle 15 minutes drain oil and remove filter. when adding fresh oil add 1 qt.Marvel Mystery oil in place of 1qt. oil + remainder of fresh oil and filter.
I have done this with 3 cars, Chevy, Volks,BMW and it has worked with each.
This problem usually happens when oil isn't changed often enough when you drain the oil after the flush you will probably see globs of sludge in the oil.
Good luck
Change it hot and change it often is a good rule.