Tru-Oil is Linseed oil... it's just like Danish Oil, a blend of linseed, turps and various other bits and pieces (Danish Oil is more or less an equal parts mix of linseed, turps and beeswax) Adding more linseed probably won't make any difference other than to dilute and take longer to cure, but you never know till you try!
I tried fletching tape once, and while it's pretty good on the thin shop-bought turkey feathers, it's not much use with war arrows that meet EWBS spec, as they need to be fletched with goose, and goose primaries are too strong for fletching tape! They just pull away from the tape as they don't want to straighten and the adhesive isn't really strong enough. Nothing beats a good bead of cheap superglue