Many good windmills have been destroyed by using the wrong oil. Each windmill company will specify the correct oil for their gearbox. Most use a 10-weight or very light nondetergent-type oil. The thin oil travels easily to the many moving parts. Nondetergent oil allows any contaminants to fall to the bottom of the gear case. Adding detergent oils will cause build-up and contaminants to block small oil ports. Once the ports become blocked, the lack of oil to the bearings will lead to major failures.