Go for a 20 minute drive (to get the 90 weight warmed up). Put a pan beneath the drain plug, put your 1/2" ratchet head (the square end of the ratchet, without a socket) into the plug and unscrew it, normal, righthanded thread. Same for the fill plug (the one halfway up the banjo) give it a few minutes to drain out. Fill with 1 3/4 quarts of 90 or 1 1/2 quarts 90, and a pint of lucas gear oil. While you're under there, unscrew your axle breathers and check to see they aren't plugged up. Before you dispose of the old gear oil, take a magnetic tool wand, if you have one and swirl it around in the oil for a minute, and see if you collect any steel shavings. Hopefully not!