I'm a salesman for Texas Refinery Corp. Do Moly engine oils and gear oils work, yep, they greatly reduce friction, wear and heat. Would I suggest a "Moly" additive for use in a hydraulic oil system, probably not, at least not as much as I would for other systems. The additive you want to see a large amount of in hydraulic oil is zinc and depending on the exact type fluid, Sulfated Ash and Calcium, not moly. Moly works great in engine oils and in gear oils where you are trying to prevent metal to metal sliding friction under extreme loads but low fluid preasure. Hydraulic oils and hydraulic systems pose some unique lubrication needs and problems that are best handled with zinc rather than moly.
Best practice is that you should start with a high quality lubricant and never have to "add" to it. For high quality hydraulic oils you should look for one that has a zinc contnet of 0.10 Wt. % and in universal tractor fluids they should be around 0.20 Wt. %. Most every day quality hydraulic oils have around .05% or less by weight and average universal tractor fluids around 0.10% or les by weight.
The only time I recomend using a additive is if your trying to boost the performance of a lower quality lubricant. High quality lubricants are enginered with (how should I put this) all the "good stuff" already there, so there's no need to add more additives to it.
The best advice is to spend the money on better lubricants at the start instead of trying to boost their performance later after problems arise.