Hi. Varsol is the Canadian brand name for mineral spirits. (Use the word Varsol on a U.S. forum and they won't know what you're talking about.) Mineral spirits are petroleum based.
Turpentine is distilled pine sap. Don't see any reason not to use turpentine but I've never used it. (It has medicinal uses too, not just as paint thinner. Haven't done those either. snicker.) It might give you a lightly different colour finish. Don't know for sure.
Pure Tung oil is used without mixing it with anything. Doesn't hurt anything if you do though. I've never mixed BLO either and it works just fine. The difference depends on what it's use. Tung oil gives a hard, waterproof, shiney, finish that usually only needs one application(that being several thin coats, using a clean lint free cloth with every coat, and 24 hours drying time between coats. About 5 coats seems to give good protection to wood.). Any stain(Minwax will do nicely) must go on before the Tung oil as the oil keeps everything else out to about 1/4" deep.
BLO gives a flat finish and requires reapplication as required. It does keep much of anything out. It was cheap and easy to get when it was used more commonly. Comes from flax plants. Same plant used to make linen.
Anyway, these days BLO is a it more expensive than turpentine. $13.99 vs $9.99 per litre. It's way less expensive than Tung oil. Tung oil is running close to $20. Personally, I use Tung oil because if gives better protection.