Easy or difficult maintenance (varnish or oil)
Whatever you use to protect your woodwork will determine how much effort you have to put into maintaining it.
Many will apply varnishes (e.g., polyurethane) and will enjoy instant gratification, only to regret it later, when deterioration of the finish requires that it be removed and a new finish applied.
Using what is called a long oil finish can reduce this problem. Long oil is just a term used to indicate the varnish has more oil added, which tends to make it more flexible and tolerant of expansion and contraction of the wood better. The down side is it is also less durable.
Because something says it's a spar varnish doesn't mean it's suited to the task of protecting such objects. It's great advertising though, and sells a lot of product. Likely to the later dismay of the purchaser.
“Teak oil” and “tung oil finish” are advertising gimmicks. To my knowledge, no one is out there wringing out teak trees for their oil and putting it in bottles. Tung oil finishes often have no tung oil in them. More often than not, they are made with linseed oil.
Both teak oil and tung oil finishes are what is called a wiping finish. This is a fancy name for thinned down varnish. You could get the same thing by buying a quart of product and thinning it fifty percent or more with mineral spirits, turpentine or other thinning agent.
I believe there are some very high quality finishes out there, but they aren’t cheap (at least one worth looking into was mentioned above). As well, application may not be as simple as just grabbing a brush and throwing on a finish.
Oils require more frequent application than varnishes, but don't require removal of previous applications before applying new ones. Cleaning should be done before application, of course.
Linseed oils, or oil based products that rely on it are food for bacteria. Not so tung oil.
When buying tung oil, it should say "pure tung oil," or "raw tung oil." This indicates you are getting the real product. Otherwise, you are probably getting linseed oil.
A quick note on things like "lemon oil": Essential oils and de-limoneen aside, there is no such thing. The lemon oil sold down town and intended for wood care is just mineral oil with a lemon scent.
Both tung oil and linseed oils harden, after the solvent has evaporated, by polymerization (pay attention to that term when seeking expensive products promoted by manufacturers).
When applying a hardening oil, it may be preferable to thin the first applications with as much as 50% paint thinner or turpentine (you can use a little more thinner in cooler weather to reduce viscosity and to maintain penetration capability). I like to keep applying the thinned batch, until the wood quits soaking it up. I may distract myself throughout the day, coming back to the project to apply more applications, as the former application absorbs into the wood. I, then, allow the solvent to evaporate off for a few days.
Once the first applications are cured, I cut back to 25% thinner and repeat the process used with the first applications. Finally, I apply oil thinned only enough to get it to apply (e.g., 5%-10% solvent).
In the end, I'm going for saturated wood. My theory is based on experience with various wood types subjected to various conditions. For example, a piece of wood may come in at fifteen percent moisture content, but will dry out more. As it dries, it shrinks, often resulting in splitting and cracking (yes, even teak). This is seen often in common household decks built from cedar, redwood and treated lumber. Oils replace moisture and liberal applications can swell the wood, over time, making small cracks and separations disappear again.
Wood full of oil isn’t going to take on water and it isn’t going to rot. However, unless something is added, the oil will not protect the wood from the effects of ultraviolet.
When it’s all said and done, it’s worth your time to learn what you can from others with experience. I’m not a boater, sailor or otherwise nautically inclined, so am probably missing a lot of tricks. A good example of the fallibility of taking an “expert’s” advice may be seen by that a lot of people have boats with expensive epoxy finishes that did not hold up and that require a lot of work to correct. They were put on my “experts.”