Welcome to the forum ....
Welcome and glad to have you here. First, is your oil injection tank still in? That tank must remain in the ski. Even going premix, it supplies oil to the rotary chamber....
The premix ratio is 40 or 50:1....but you must make sure you use the right type oil. It must be a non-NMMA, non-TCW-3, low ash oil. Any other oil will blow your motor...........
Thanks for the question........
Oh, you can buy it at Wal-Mart....I buy it by the gallon. Buy the quicksilver blend and make sure on the back, it states it meets OEM Sea-doo PWC requirements.