[Rimrock]

If not having the grade 22 oil at hand, it's better to chose a ticker variant, like grade 32 which is more common and can be bought in small containers. (There might be some mechanical shops / industry in your local area that purchase the GLP22 in barrels and other big containers, that might let you buy some small quantity if one check around)
One should naturally in general follow Premacon, and other suppliers, advice when it comes to oil grade, but on a scale model with industrial standard hardened steel pump and valves there should be no big reason to have sleepless nights if operating with a thicker oil for a periode, IMO.

The fabricator of the pump that Premacon, and some others use, Jung, actually give grade 46 as the standard in the data sheets, which is one of the most common used grades in the 1:1 life. I would think that due to the small channels in valves and pipes/hoses used in scale modelling, on have to compromise and choose lighter oils in order to get a better fluid flow and reduce power consummation. Maybe an issue here is the risk of wearing out the pump motor, rather than ruin any of the hydraulic components, when using a thicker oil, and less motor wear with the thinner oil.
Someone should ask Premacon what could go wrong if not going for the grade 22)