1) Check with the manufacturer for directions on how to remove the oil and clean the teak before applying the final finish.
2) Instead of varnish on the interior, think about using something like MinWax Quick Dry Polyurethane. I've used that on the interior of two boats and really like the look -- and it's easier to apply than varnish, holds up better, etc.
Sailingfool tells no lies, if you ever have to redo the interior varnish it is a chore you will hate worse than the job of doing it the first time. What I did on my old boat was varnished the trim and left the walls oiled which looked great and I did not have that much to keep up with. The boat I own now is oiled. I am going to clean the wood with the Min Wax cleaner,sand to remove scratches and fill a few screw holes. Then when all this is done I am going to try an old method of a high gloss finish used by gun makers "boiled linseed oil". I have used it to refinish stocks on old guns it is beautiful and holds up for years I did an old Browning and it has lasted for almost twenty years now still looks great. I do not know how well it will work on a boat but worth a try. I tried the olive oil treetment wood looked beautiful but molded real bad after only a month in Florida heat. Any one have any information on the linseed oil let me know before I blow it and waste allot of time. I figure it is an oil and I can wipe it down with Amazon if all else fails.
Even though some of the hard-core varnish junkies here will bash Cetol products, most will agree that Cetol over exterior teak is the way to go. Also, I'd guess that 75% of the 400 teak-trimmed boats at my marina choose the Cetol route. This is especially true for boat owners who value sailing more than hanging at the dock, spending nice days applying 12 coats of varnish to their brightwork, sanding between each coat, then recoating every 3 months after . . . more often in the tropics.
In addition to natural teak decks, my boat has a forest of exterior teak. Cap rails, toe rails, pilothouse doors, trim & eyebrows, aft deck seat boxes, pulpit and a very large sliding hatch over the pilothouse. With this much brightwork, I decided to completely strip all old varnish two seasons ago (contracted this out to Antiguans) and initially applied four-five coats of Cetol Light. After which, one maintenance coat is required every season. No sanding between coats is needed, only a light scuffing with a ScotchBrite pad before the annual coat. I am still debating over whether or not to top coat with clear Cetol gloss coats.
I really like the Cetol Light hue over the darker Cetol Marine. The finish is similar to varnish, although initially, somewhat more light orange in color and more opaque. I have found that the finish is softer than varnish, but abraded areas are very easy to touch-up with disposable chip brushes and blends in well. No regrets with this sailor.