What Is The Story With JD303 Hy-Trans Oil?

Most Yanmar tractors require a specific oil for use in the transmission & hydraulic system. For many years the recommended oil type was "JD303" or "JD303 Compatible" oil. JD303 has been the go-to oil type since about 1960 when it was introduced. It was in production up until 1974 when it was banned by the US government because it contained whale oil. For many years after that the "meets JD303 specifications" was used by numerous brands and everything was fine. The only problem was that the actual 'standards' were lost along the way and, without any actual rulebook to go by, some oil brands were substituting much lower grade lubricants to save money. 

So what oil should you put in your Yanmar tractor? JD303 has been replaced by J20. If you look for an oil from a trusted brand that "Meets J20" standards" you should be just fine.

J20 currently comes in three variations.  There is J20A which is an older, discontinued, formulation that is considered to be a very close substitute for the 303 oil. It is an older formulation that has been replaced by J20C which offers better protection for gears and has additives to reduce oxidation. J20C is usually the best oil for your older Yanmar tractors and works in most climates as it is recommended for temperatures between -13F and 122F.  J20C is also used under John Deere's trademarked Hy-Gard brand. There is also a J20D which is recommended for areas with much colder temperatures averaging from -40F to 85F.