80W90 is the right viscosity. The final drives and the transmission are separate and do not share the oil supply. Flushing the tranny with diesel or kero is always a good practice when changing the gear lube.
As a final note, the other suggestions here are fine, so use what ever gear lube you have, as long as it is new and clean. The heavier weight lube might be a bit too stiff in cold weather depending upon where you live. People get a little too indigestive around oil and gear lube for these old girls. They like CLEAN oil and lube, changed appropriately. Unless you are constantly working the snot out of your letter series tractor, clean oil and new filters are the best thing you can do for them......................much more important than what brand or multi-vis vs. straight weight vs. detergent vs. non-detergent. Keep 'em clean and keep 'em full. You will be just fine.
This post was edited by Tom Fleming at 03:14:27 12/05/12.
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