As much as I wished there was one finish to rule them all, there isn't. You don't need to refinish a varnished paddle if it gets damaged, just like an oiled paddle, it gets some maintenance, albeit, it is a bit more work. Routine maintenance is probably one of the most important items for gear longevity ... probably more important than the type of finish IMO.
As stated ... oil has it place with certain paddles and it works very well, for example, for the carved paddles you are making. If on the other hand you decide to oil up your new composite wood, epoxy/glass coated blade, with an oil finish, well, it just isn't suited to that task. There are a lot of flavours of DIY wood paddles being made and finish selection needs to made with product protection requirements in mind.