John,
Is heavy oil really necessary? Ferguson tractors used a "tractor universal" oil of SAE grade for the engine and rear axle which inlcuded the hydraulic pump and they went forever. AGCO dealer here sell the same sort of thing now reccomending it for the older 50s type MFs.
I would have thought that an oil in the 30-50 range would be perfectly OK for the odd hour or two these old brutes are ever going to do these days???
I put 30 oil in my green Challenger and it is fine. Don't forget that modern oils are of infinitely superior quality to the old ones and after all you are only seeking some lubrication for very short periods of time.
No doubt someone will get violent and contradict me!!!!!!
John
John