Dulling semi-gloss oil base BEFORE using it
In a way, it's kind of refreshing to hear that someone actually prefers an oil-based product over the easier-to-use latexes...and there are still scenarios where oils out-perform latex coatings. Personally, I wouldn't use it, or recommend it, in a bathroom (especially one with shower), but for trim, walls in need of extreme washability, or areas subjected to moderate abrasion or scuffing, it's an excellent choice....
Now to answer your question. Yes, the sheen of an oil based product will "tone down" over the next few weeks after application, but only enough that some of the harsh glare may disappear, but it'll still be a semi-gloss. Adding thinner to an oil based paint really won't result in a lesser sheen and may actually make the finish appear a little shinier due to the coatings ability to flow to a smoother finish with the addition of thinner. You may be able to find a paint company that still offers packaged flatting agents, but be careful with those - they will soften the finish beyond what you might wish for. Typically in years past, manufacturers made a flat enamel (oil) that was really closer to an eggshell finish, but good luck finding any of that in California. I also wouldn't mix different brands of oils (with different sheens) to try and accomplish a more satin finish. There are just too many variations of alkyds and their new solvent structures to suggest that different brands would be compatible.
For your next job, if you want to stay with a low-sheen oil (alkyd), you may want to look at your dealer's rust-inhibiting line of alkyds. Most manufacturers make at least a white in a satin version and these product will work fine on interior walls and trim, although you may be a little limited in color selection.
Good luck, I wish there were a better solution for you.