I like to use synthetic GL-5 or better gear oil in Rotary Cutters and MMM gear boxes. The synthetic oil tolerates heat better. Gear boxes on a MMM or Rotary Cutter get hot. Most Rotary Cutter gear boxes have a vent hole and during prolong use the oil inside will get so hot it will percolate out the vent hole. Some will also say the extra money spent on synthetic oil is wasted. Each equipment owner I feel must make there own decision what level of protection they feel comfortable with.