The engine oil drain plug is beneath the engine on the oil sump or oil pan. The plug under the gear lever on the bottom of the transmission case is one of two plugs you would undo to drain the transmission/differential of oil. The second plug is a little farther back towards the rear of the tractor. There is another plug on the right side of the transmission case located in between the two transmission drain plugs. This is used to check the level of the oil in the transmission case. Oil should just drip out of this plug hole when the transmission has the right amount of oil in it. It should take 10.8 liters or 19 pints (Imperial) if completely empty. The transmission oil is filled via the large round plug on the top of the transmission case right near the shift levers. Be sure to fill slowly with the level plug removed and get just the right amount into the transmission. Do not overfill. The oil to use is a good 20W/50 or 10W/40 Multi grade such as Castrol or Shell. Leyland recommended that the same oil be used in the engine, transmission and the hydraulic system. If your doing complete lubrication don't forget to drain and refill the oil in the final drive gearboxes located at the rear wheels and while your at it you may as well change the oil in the hydraulic tank as well as both the engine oil filter and the hydraulic oil filter.
Good luck!