I was looking at a boat and the owner told me i would have to premix because he removed the "oil sender unit" ( i think that was the wording) off the merc 115. should i be worried about this? Is premixing expensive pain? again im a newb an sorry if a newb question.

He just bypasses the VRO,which is an oil pump..Premixing while takes a LITTLE measuring, takes the worry about the VRO going out and burning the motor..Nothing bad about premixing, unless you forget to add oil...
the VRO meters the oil, as the RPM's dictate, so if you permix and do a lot of trolling, idle speed, you might need plugs sooner..not a big deal in most cases..

Thanks I just wanted to make sure it didnt mean somthing bad.

If your looking to buy the boat, He has probably
already done you a favor,

I would do a compresson test before I went too far with this ...

I've got a 2005 Merc 115. The onboard oil tank, "pump", and warning system are a joke. The warning horn only tells you if there is a low oil condition. Problem is, if your oil tank is topped off but the engine isn't receiving oil, you won't know it until the engine cacks--there's no warning for proper oil distribution. When the system works properly, the mixture will change to something between 80:1 and 100:1 at low speeds. That, as kingdad 101 said, could give you a bit longer plug life. Hardly worth it considering the price of a new powerhead. For my money, you're better off mixing the fuel yourself.