94 Seadoo XP--
I want to change brands of injector oil from the cheap TC3W the ski came with to a API-TC oil. Do I have to completely drain the old oil out of the lines or just in the oil tank? both oils are mineral based. Thanks for your help..
Anytime you disconnect any part of the oil system, i.e. lines or tank draining, you have to prime the oil pump to get out any residual air after refilling. There's no way way to do it unless you want to cavitate your oil pump.
As for the oil that's in there now, burn the rest of it off until you have about a1/4 tank and fill it back up with the good stuff and you'll be fine.