Yanmar transmissions don't hold much oil. My 3GM30F engine with a KM2P gearbox holds only 0.3 L or about 300cc's or 10 oz. I use the same SAE 30WT in the gearbox as I do in the engine.
Check the level by removing/unscrewing the dip stick tube, wipe clean then reinsert the tube. Do NOT screw it back in. Just rest the bottom shoulder of the screw cap on the top edge of the hole. Then remove and look for the oil level. The fluid should just reach that little line near the bottom of the tube - that's it. A Yanmar engine school tech told us to spray some white paint on the bottom of the tube to be able to see the fluid level more easily. My Tranny takes 30W engine oil but your's might be different. There should be a manufacturer's ID plate on the transmission with info about whether to use engine oil or (ATF) transmission fluid.
A trick for checking the transmission oil level, that a mechanic showed me, is to use a clean paper towel and rotate the diptstick on the towel to visualize where the oil level is in relation to the groove on the dipstick. It is definitely difficult to see the oil level on the stick. I always have a paper towel in hand any way to clean up any drips and to clean any debris from around the cap before opening to remove the stick, so it's easy to use the towel to help with reading the stick level. Test your paper towel for wicking and absorbancy.
- Paint dipstick white to better see oil level. The paper towel trick also works fine.
- Make sure the reading is taken without the dipstick threaded into the hole.
- Fill with 0.3L (300cc's or 10oz.) of 30 wt oil
- Don't overtighten the dipstick. I've never had an issue but the plastic has been known to fail.
