Mixing oil based and water based paints will not give you excellent results but disastrous ones because oil and water do not mix and therefore these types of paints cannot be combined. Water based paints cannot stick to oil based products so it runs or peels, and oil based products cannot penetrate wood and can never set if there is water paint on top of the surface.
Oil based paints are made so that they will sink inside the wood and when you examine a buildings surface you can easily detect stains and varnishes because the wood grain is seen, is colored and enhanced by this oil based products. However with the coming out of non-transparent wood stains, it will be more difficult to determine if it is oil based because although it still sinks into the wood, their nature makes the wood appear painted because it covers the surface.
If you find it difficult to find out if the paint is oil based or water based since they often have the same characteristic, try the goof off test instead. Goof off removes latex (water based) paints easily, so you can determine if the paint is water based by rubbing goof off using a piece of rug and if it comes off easily then you know for sure that the paint is water based. Your local hardware store usually sells goof off and you can simply buy it there. To make sure that you are buying the right one, check if it says something about removing latex paint. When doing the goof off test make sure to choose an inconspicuous part of the wall. Sometimes you may need to test walls with paintings that are already peeling off but you can still apply the goof off test by making sure that it is the goof off that is actually removing the paint, then check if the paint bubbles or looks like melting.
Latex or water based paints might be ideal for your project because it is easier to use. The more popular paints in the past were oil based and that is why you can almost be sure that if you see an old building, its paints are oil based. Coating a solid oil based painted wall with a primer will make it possible to repaint it with latex or water based paint. This primer will bond with the wall and provide a surface for the latex paint to stick to. If you inform your paint distributor that you are painting latex over oil based paints, they can tell what type of primer you need for this purpose.Lessons Learned About Options