Oil Level & Change Schedule
Check your transmission oil fluid level regularly and change it following the owner's manual guidelines. Never go more than a year between oil changes. Each transmission model will have specific guidelines. Here is the general ZF guidelines for oil check and change:

• On transmissions with an oil cooler, during normal operation a certain amount of oil will collect in the cooler and lines, which will flow back into the transmission after a rest period. This may cause the oil level to exceed the maximum mark. Do not remove the surplus oil.
• To properly check fluid level, run engine at idle speed in neutral until cooler and lines are fully filled with fluid.
• Turn off the engine and check the fluid level within a short time. For dipsticks that screw in, do not screw them in to check the fluid level.
• The transmission oil should be changed after 25 hours of initial operation (with a new installation). All subsequent oil fluid changes should be every 300 hours or once per year, whichever occurs first.
• If the transmission has a filter, it should be changed or cleaned with the oil change. See filters here for common transmissions.

Oil Types
Consult your manufacturer manual for the proper oil to use. If you aren't sure what oil type to use, here are recommendations:

• For ZF 12, 15, 25, 30, 45, 63, 68, 80, and 85 series, and Velvet Drive 71C, 72C, 73C, and 5000 series, use an ATF fluid which meets Detroit Diesel Allison C-4 specifications (e.g.: Dexron IIE, Dexron III).
• For ZF 220 and larger series, use a marine 30W transmission oil (no/low detergent, not multi-grade). In very hot climates 40W is okay.