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Paul- The purpose of oil is to lubricate, protect, and with detergent oil to clean your engine. You have a tractor that is 50+ years old. Undoubtedly, it has parts which are worn well beyond original specifications. Many have been replaced or machined over the years. Others have not (most likely) usually because someone determined that replacement or machining these parts was not necessary or possible. Interconnected moving parts wear each other at points of contact. By definition, that means that they become separated little by miniscule little over time (a "gap"). Therefore, when worn, they contact with a tiny shock force. Examples are rod knock, piston slap, dist shaft play, etc. This shock force may later develop into part failure, including things such as thrown rods/broken pistons. That "gap" and the shock force can be lessened by thicker oil, which is why most mechanics recommend heavier oils in a worn engine. Better protection. Heavier oil also tends to stay in the gaps when the engine is not in use; therefore, he engine has better protection at start-up and warm-up. THEREFORE, unless you are certain that all engine parts are within original specs, I recommend using 10-40 for cold climates and straight 40 for warm (even 20-50/50 for very warn engines).
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