I have heard that once you let oil go so long without changing it, that it perhaps does more harm than good to change it. Example, a Kohler 22.5hp engine with 510 hours on it has never had to oil or filter changed, just oil added as needed. Should I change this oil? What type/weight should be used? Should I just change the filter? Note: I am not the one who didn't change the oil to begin with.