I asume you are talking about varnish?
You have to ask yourself what the purpose is to seal your paints.
Some oil-paints dry in different degrees of shinyness when you thin them down with turpenoid(however fir/resinous turpentine will not have this effect).
When you alter the oil/pigment contents in your paints by removing oil from the paints,you will end up with a somewhat more leaner matt paint.
When painting layers of oilpaints over a cut down(lean) oilpaint,..the cut down layer will absorb a degree of oil from the layer on top.
To prevent this you could seal the lean paintlayer.
Usually you want the lean layer to absorb though,..this will enhance the adhering of the multiple layers and it will serve to get a uniform oilcontent in all your layers.
You have to look at the oilbinder as a necessary evil.
Oil does yellow over time,..so as less oil as possible is preferable!
You want to keep some oil in the oilpaint though to keep the pigments binded
The more oil content in your paint, the more satin shine and vise versa..so sealing could serve to get a a uniform gloss/satin/matt
A sealer could be used aswell to protect the colours from outside influences.
A sealer/varnish can make the colours richer,..more dept of colour,..this will be very noticable when using transparent/translucent colours over a lighter ground(glacis/glazing).
A gloss sealer will make your colours apear darker than they are though.
If you desire a matt/satin/gloss"oil"paint that has a strong binder,blends well, is quick drying and does not need protection from the environtment for the comming +/-10 years and is rich in dept and colour and will almost not yellow over time,..you could absorb most of the oil from your paint and mix gloss/satin//matt resinous varnish through this paint mixture to replace the oilbinder.
You will end up with a resinous/oil paint.
I think this is a far superior paint compaired to oils alone.