We used to call that Long oil in the restoration bizz. 3rd generation finisher trained in the "Old" way's. My Dziadek made all his stains and varnishe's and we even made our own brushes,nobody wanted to bring their dog's over,they would leave with a bald spot!
The stuff you can get commercialy is fine,just dont hurry ANYTHING!!!! It's mostly about application. I'm able to apply polyurathane that looks EXACTLY like 25 hand rubbed coats of BLO. SLOW,THIN,SPARINGLY! By the way,back when poly's first came out it was like BLO only with lead acetate and flax seed oil rather than linseed oil. Hard as a rock,good only for floor's. Now it hard to say WHAT'S in it,too many chemist's involved now with all the long polymer technoledgy.(wish there was spell check!)
I thin the first half a dozen coats of BLO 1/2&1/2 with turpentine,then full strength after that.