I build 4 or 5 rods a year as a hobby so I use mostly pre made cork grips. Would it be wise to use oil on these handles or are they already oiled by the manufacture? Thank you

They are not finished at all, just bare cork. It's really personal preference on what to put on them, if anything at all. Some folks, like myself, prefer an oil or oil type finish, some like just sealer and some like nothing at all and just go with bare cork.

On preformed grips especially, I like a good gunstock finish because it locks in the fillers, and there is usually a lot of it on them, I never have to worry about cleaning the grip, I prefer the feel and think it gives me a better grip with wet hands, and I really like the color it gives. Some people feel the opposite. Try it out and see what you prefer.

Tru-oil gunstock finish has tung oil in it I believe. Two light coats and your done. Doesn't get slick in the rain, and keeps your grips cleaner as mentioned above. Wally World and sporting goods stores carry it. Finishes and stains don't pass very far into cork, so if you don't like it, you can take it off with a few minutes work.

Tru-oil is a linseed oil derivative, meaning very highly processed. There may be some microscopic amount of tung oil in it, but I wouldn't count on it. I use U-40 cork seal, one coat, wipe off the excess and you're done. It doesn't get any easier than that.