Raise the engine and use the support lever to lock it. Check the fluid level to see how much you're actually losing. Chances are it's one of the 'o' rings that's failed. By all means use the stuff jyasaki recommends. Never tried it myself, but the only other option is a full strip down and rebuild.
Parts are itemised here. Go to page 74. http://www.tohatsuoutboardparts.com/...0221041-4).pdf
You can order from main Tohatsu dealer plus small delivery surcharge from Tohatsu as they rarely carry these items in stock at dealer.
Top up fluid is Mobil ATF 220, then purge system of air, etc.