Well.. my take is that given the long service interval, cost is not really an issue. At lest for me, I do not mind dropping 10-20$ per quart, esp if that buys me some extra longevity and improved reliability . Trying to save some money on the lubricant is simply not a consideration for me.
The per year cost works out to something manageable in any case.
The hydros are a high temp, high shear environment. There is no combustion, so acid neutralizer (TBA) content is probably not relevant.
No cat converter, so high anti-wear additive levels are OK. My sense is that high viscosity index is probably not important, since, the
typical use is not cold temp. The very high viscosity indexes are usually achieved by friction modifier additives, which
tend to shear down to some degree. So I would stick w thew 20w50 recommendation, unless there is a good
reason for a lower winter viscosity.
I am thinking: a high quality true synthetic 20w50 , with maxed out anti-wear additives. So on that count the redline motorcycle oil sounds
pretty good to me. The ester content in the base stock should help eliminating carrier ( usually dyno) oils for the additives, as it is an excellent solvent for those. Expensive, but at least you know what you are getting, instead of just some marketing claims such as "superior performance" .
I don't much care of the "Cant tell you what base stock we are using, it is a trade secret".
Seriously? For my hard earned money, I require that a manufacturer goes on record and discloses what base stock(s)
are used in their product. No disclosure, no buy. Esp. if I see from the MSDS that they are using a significant amount of
the cheaper group 3-s yet charging top $$$. So in general I mistrust the highly hyped/marketed boutique oils such as Royal Purple
and Amsoil. I could be wrong and those may be actually fine products, but simply, some key info is withheld.
A low "W" oil, (to me) implies 2 things: Better cold starts when it is very cold, and a higher viscosity index. I do not believe either is beneficial in the hydro/zt use case. Because of some amount of shear down, esp. over the expected many year service interval, I would like to have some extra margin on the 120 celsius cSt number, so after a few years it still meets the minimum recommended. All this so far implies a high quality, true synthetic oil. The 20w50 redline motorcycle oil does actually look good to me. I am on the fence on whether the oil should be detergent. Since there are no ashes or other combustion blow-by, I am thinking "detergent" is not relevant. You cant go too wrong with a M1 20W50 either,
where the base stocks are PAO+GTL these days.
I recall reading on this forum, that the reason for the motor oil preference is the high heat operating environment. Something about gear lubricant being too thick for this. For example I read the a sometingW60 oil is worse for a very hot application than a W50.
I dont claim to be a lubrication expert, just tabulating the data from what I am reading.