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Yanmar Transmission Oil Level

May 27, 2004
Hunter 30_74-83 Ponce Inlet FL
Does anyone know how much oil should be in the transmission of a Yanmar 2GM20F.
The dipstick on mine only shows one groove at the very bottom... No typical min/max lines.
The cap/dip stick looks original and the manual I have does not list a volume that I can find.
Thanks in advance.
Jun 4, 2004
Hunter 340 Forked River, NJ
The small Yanmar engine tranny's don't hold much fluid. My 3GM30F tranny holds only 0.3 L or about 300cc's or 10 oz.
You check the level by removing/unscrewing the dip stick tube, wipe clean then reinsert the tube but 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 Trans with info about whether to use engine oil or transmission fluid.
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Dec 25, 2000
Hunter Passage 42 Shelter Bay, WA
Spot on, John. Lots of confusion about that, too. Our current boat tranny plate (4JH2-TE, KBW20 Kanzaki-Carl Hurth reduction reversing gears) is mounted on the bell housing connecting the tranny to the engine. It says ATF. When I removed the old oil late this summer it was clearly engine oil placed there by the PO. I replaced it with ATF. As I recall our 2GM20F (first boat) took engine oil, but I never checked the tranny plate to see what it recommended.
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Jun 4, 2004
Hunter 340 Forked River, NJ
Yup! I corrected my post since 0.3 L is 300 cc's but check the level when adding fluid since it's a bad idea to overfill the trans and then you have to drain or pump out the excess. I think I meant to say that 0.3 L is approximately 10 oz.
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Jul 24, 2005
Beneteau 323 Manistee, MI
Depends on which transmission. KM series uses engine oil. Others do use ATF.
Jan 7, 2011
Oday 322 East Chicago, IN
I have a 2GM20F Yanmar in my boat, with the KM transmission. Mine also has a single groove near the bottom of the stick (which is a short, stubby stick). I use the groove as the indicator.

As others have said, my tranny uses oil, same as the engine, not ATF.

May 27, 2004
Hunter 30_74-83 Ponce Inlet FL
Thanks guys. I'll be verifying the level Sunday and adjust as necessary.
Seems like a small amount though.
Nov 6, 2006
Hunter 34 Mandeville Louisiana
most of the 2 GM 20's had a KM2 transmission which takes 250 cc of engine oil, one liquid cup , or 8 fl oz


Nov 4, 2010
Beneteau 331 Deale, MD
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.
Good tip there, John. I find reading the oil level on that tiny scrap at the bottom of the stick to be very difficult. I'll try the paint.
Aug 16, 2009
Hunter 1986 H31 California Yacht Marina, Chula Vista, CA
I assume that when folks refer to using "engine" oil in the tranny they are referring to the diesel oil used in our motors? Besides the claim that synthetics can run longer, do they offer other advantages? I did an abbreviated search and found this informative quote from Cox Engineering provided by Charles Erwin:
"Yacht auxiliary engines generally run for short periods, at lower temperatures than any other duty, and typically in the lower half of their rev range. From a lubricant point of view this is a very undemanding duty, accounting for their low API requirement. Combustion temperatures never reach levels at which a high-performance lubricant can be effective, so the overall chemistry in this area is not neutral but alkaline (basic). There is a great deal of evidence to show that use of an oil with a TBN that is too high for the duty can lead to several problems, particularly high wear rates of cylinder bores. The effect of high TBN engine oils in accelerating wear rates is well known. Burning high TBN oils may generate excessive deposits on the piston crown. As the piston tilts when moving up and down these deposits become compressed and will form a very hard and abrasive layer, causing excessive wear, mainly in the bore."
I also found a contrary view from Marc Robic:

"Being in the automotive industry for the last 20 years, I can tell you they synthetic oil technology has greatly improved. For example, concerning seal and gaskets, today's quality synthetic oil have additives that will actually help improve seals and gasket performance. But because synthetic oil is thinner, if a gasket is somewhat already in poor condition, it MAY leak. The good news is, as stated before, many higher quality synthetic oils offer additives that will help older gaskets swell and restore any prior minor leaks. What is good to know, is that the medium and long term benefits of synthetic oil is the reduced wear and tear on engine components, easier starts, better engine preservation during those long winter months (for us Canadian sailor..). However, be careful of the myth that using synthetic oil will prolong the oil change intervals. Although. Synthetic oil will not break down as fast as regular oil, the oil change intervals recommended by the engiene manufacturer must be respected. Lastly, for those that may not be aware, if you want the full benefits of synthetic oil, make sure you also use a synthetic oil filter!"​

I don't use synthetic in my 2GM20F because I am nervous about less frequent oil changes. Might there be any advantage to using/switching to synthetic oil in the transmission?
Jun 21, 2004
Beneteau 343 Slidell, LA
The specs for the 3YM30 transmission is 0.3L (0.32qt) of SAE 20 or 30 wt motor oil.
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.