I'm finishing a hard maple joiner's mallet with boiled linseed oil (BLO). The instructions on the container say to cut it with 2 parts mineral spirits to 1 part BLO. Is this strictly necessary? From what I've read, thinning the BLO will let the finish penetrate the wood deeper, but I'm not sure whether that's necessary for a mallet which will get beat up either way.
No you don't need to thin BLO, or any other drying oil, to apply it.
From what I've read, thinning the BLO will let the finish penetrate the wood deeper
That's the theory! But it's one of the most persistent myths in finishing. In reality it doesn't appear to make any real difference (except possibly in end grain).
People have run tests that have shown that it increases penetration and other tests have been run that show zero difference* and it's important to note that where deeper penetration was measured the improvement was minuscule. That kind of thing might be important to a luthier working in thin sections of wood, but for a bulk item like a piece of furniture or a mallet it's hard to imagine that an extra 0.1mm is going to make any appreciable difference!
From trying it both ways I can say that, empirically, the wood looks exactly the same, there's no difference in durability that I can notice and the surface feels no different, as long as you go on to apply enough coats in total — there is a massive difference in all three of these if you wipe on three dilute coats of BLO versus three undiluted coats.
Another option you might like to experiment with is to gently heat the BLO to reduce its viscosity (best done in a double boiler or some similar arrangement, not over direct heat). This supposedly increases penetration too. If you try this it's quite evident it does reduce viscosity, the oil is visibly more runny, however once applied to the wood it again appears to make little or no difference in how deeply the oil is absorbed. I'm actually a fan of doing this, especially during cold weather, but only as a means to easing application. I don't delude myself that the oil is going in any deeper.
*What these tests may inadvertently have proven is that the type of wood is a key factor in relative oil penetration, but that shouldn't have been any surprise anyway.
Graphus gave an excellent answer. Once, having asked the same question to a guy very experienced in wood finishing, he explained to me that thinning the oil doesn't make oil molecules any smaller—it only spreads them further apart. Molecule size determines how deep it will penetrate and, like Graphus pointed out, an impression that thinned oil "should" penetrate deeper is just that—an impression. You only end up with unevenly-distributed oil, which is why it requires several more coats to reach the same effect as with pure oil.
note of advice: BLO contains added trash that makes it cure faster, it's not a pure oil. It also yellows and darkens over time. I always use Hope's pure tung oil and I've had much better results.