Bob~
Although I used oils for painting gunners in my youth - mostly Herter's and Parker's pre-mixed, I am new to tube oils on decoys. (Significantly more experience using oils on flat work.) I have been learning for several years - but always on my "mantel-piece" birds - and not my gunners. I will occasionally use flat oils (alkyds like Rustoleum) as topcoats. I especially like the FME (Flat Marine Enamel) Black Duck body color that Lou Tisch sells. You might want to base coat your birds with that - and use the tubes for the face and bill and other lighter areas.
I like to seal with spar varnish and then a flat oil. This works for either latex or oil topcoats. However, some who use strictly oils prefer the traditional approach of linseed oil as the sealer. I will defer to those with more experience than I.
A big part of mastering oils is mixing the media - oils, thinners and driers - to work as you want them to. There is a bewildering array of products in any art store. Of course, durability under gunning conditions is seldom a concern of the purveyors. Pre-mixed colors usually have driers added and can be relied upon to cure overnight. Pigments in tubes, on the other hand, dry at varying rates (whites seem to takes forever....) and so you'll need to vary your media with each pigment. Similarly, different pigments dry with different lustres.
So, this is probably not terribly helpful. I will let others with more experience weigh in. I do know that I like the time that oils give me to blend - but am also frustrated by the longer drying times.
One last word - a bit of wisdom that I learned from others on this site: If you use tube oils, spend the extra money for the best stuff - same for thinners, driers, etc.
All the best,
SJS