The factory fill oil is a high zinc oil with an ideal synthetic base viscosity which requires very little viscosity modifiers and therefore a much more stable oil. The viscosity is classified as 15w-50 but what makes it especially great is that it sustains great viscosity at high temperatures and contains a high level of zinc phosphorus. This oil should not be changed out on new machines after break-in and should remain the system for the first full interval. This oil is compatible with Mobil 1 and may be mixed. We offer this oil by the bottle and upon request we may sell it by the tote (275 gal). This oil will maximize the change interval and extend the life of the drive system in nearly all lawn mower hydrostatic applications. Although we recommend 500hr oil changes, our testing has shown that this oil retains the required properties beyond 2000hrs.