If I wanted to prep with oil, I would use Deks Olje #1, following the directions on the can for applying multiple coats, wet on wet. It will be dry in a day or two, unlike linseed and it doesn't turn back in time like linseed oil can and does. However, I'm not at all convinced that such a base for your varnish is a great idea or likely to be an improvement. I once had an Old Town Trapper in my store and wasn't thrilled with the interior finish on this particular canoe. I called the factory to complain and was told that it had the "usual" schedule of finishing for all of their wooden canoes - one varnish coat thinned 50/50, followed by two coats at full strength. Granted, you can certainly improve the cosmetics with a couple more coats than they used, but in general, that scenario worked pretty well. I don't see much reason to do something different or add additional materials to the mix. As for thinning, heavily thinning the first coat is to improve penetration a bit. Thinning following coats should be done if and when it is needed to improve the way the varnish handles - on that particular day and in those particular conditions with that particular brand and type of varnish. If the brush is dragging and the stuff isn't flowing well, by all means, adjust it a bit, but it is better to adjust paint or varnish to the job as needed than just blindly or automatically.
