Outboard Motor Oil

I have a 99 merc 115 that has been on synthetic blend for as long as i’ve owned it i’m all out of the blend and was wanting to switch over to full synthetic outboard motor oil, my question is do i have to drain the existing blend remaining in the oil reservoir OR can i just start topping off the reservoir as i use up the oil, next i was wondering is if anyone has hooked up their trolling batteries up in parallel if so how did you do it and how does it work for a 12V motor- thanks!