I have a point or two to bring up if you don't mind.
I built a glider, a porch swing and a picnic table out of aromatic cedar which is an OILY wood. I gave all pieces three or four coats of "Helmsman Spar Urethane" varnish which is advertised for "Exceptional protection from sunlight, rain and moisture and temperature changes". All items are outside and the finish has lasted about two years and is falling off in spots already.
My point is....I wouldn't put any oil on your project BEFORE the finish AND I would make sure that the spar varnish is for MARINE use.
I truly believe that the oil in the wood I used caused the finish not to last as long as I expected. So, I wouldn't recommend that you ADD oil to the wood before finishing.
Also, I'm sure that the spar varnish that is designed for the average wood worker is not as good as that designed for boat builders (I may be wrong on that point) Ask the person at the paint store if there is any difference in spar varnishes.
I may add that I live in Kansas USA which is known for extreme climate changes through the year and your location may not be as harsh on things out doors. My patio where the items are, ranges in temps from -10 degrees f in winter to +120 f in summer and are exposed to harsh sunlight, rain and snow depending on the season.
Rog