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I'm a retired engineer and have worked Farmall M's for twenty years. The factory recommended twice per year oil changes as a minimum primarily to get a lighter weight oil in the engine for the winter. Since the engine was designed to give long life on what is now a subatandard oil, the only thing you should be concerned about is water contamination from too much time not running. I think synthetic oil is a complete waste of money in these old, reliable engines if they are stock or nearly stock. Break in with the cheapest oil you can find to get the system cleaned out then use the multiviscosity oil of your choice and you will run a Farmall for many many years. I use 20w50 in all the tractors including the Liftalls. Farmalls have very low speeds and low pressures combined with low temperatures that make the automotive advice superfulous. I believe changing the filter only in an engine without a full flow filter is probably not a good idea. Most my tractors get new oil once per year and use very little oil under hard pulling and no oil during light work.
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