Romke, very observant post
However true in the past, most pump manufacturers had been forced to cover themselves as water-glycol technology wasn't what it is today. The slight viscosity differential from a mineral oil fluids are negligible. Each batch (Mineral, synthetic, wg) will vary by slight amounts depending on the add-pack, baseoil viscosity used in the blends. The large bonus of water-glycols are that thye are synthetic in every sense of the word, that being said, they have a huge VI in comparison to any standard mineral oil on the market, therefore, during operation, the viscosity will not change at the same rate of a mineral oil and most systems will run better with less component degradation.
A good question asked is what the system contained before the wg fluid. If it was a mineral oil, and was flushed properly there should be no 'issue' in theory as the mineral oils wouldn't be significant enough to damage the new fluid. However, if there is a significant amount of mineral oil residue, it can wreak havoc on the fluid and in turn the system. Simple monthly (or more frequent) checks would be able to distinguish if there is residual min-oil causing the problems.
If the system was new even, you may want to check with the manufacturer of the system, to see how they test before shipping, in 90% of shops, unless specified a standard mineral oil is used as they are cheaper.
Seals/Elastomers:
Most 'newer' seal mediums will handle water-glycol fluids as the technologies have grown with the growing popularity of water glycol fluids. Unless you're using 'old' seals or something that is specifically not supposed to be used with water glycols all should be well.
Most suitable seal mediums are good for many different fluids, these are some recommended for use with water glycol fluids.
Suitable Elastomers
Fluorocarbon (Viton, Teflon)
Silicone (Silastic)
Fluorosilicone
PNF (phosphonitrilic fluoroelastomer)
Ethylene-Propylene
EPDM (ethylene-propylene diene terpolymer)
Natural Rubber (polyisoprene)
Butyl Rubber (isobutylene/isoprene)
Buna N (acrylonitrile-butadiene rubber, Nitrile, NBR)
Neoprene Rubber(Chloroprene)
These are NOT recommended.
Unsuitable Elastomers
Buna S (styrene-butadiene rubber, SBR)
Polyurethane
Epichlorohydrin
Other polar elastomers
Leather (dynamic seals)
Cork (dynamic seals)
If any further questions comments or concerns let me know.