Indeed it is made from linseed oil, but uses a different process of partial polymerization. It was known as stand oil in years of yore. Because it is partially polymerized by heat in the absence of oxygen, it completes the process of curing differently than the reaction with chemical dryers in BLO.
I believe spar varnish is oil based.
I have tried every finish known to man, and some concoctions that I created myself. The easiest and the best I have found are the oil based varnishes. Next would be spar varnish using a dipping method, but it can be a pain to get consistently until you have some experience at it.