[ptebbe]

I think that the lower viscosity oil flows easier thru a model's smaller hoses and connectors and therefore works the pump less and also increases efficiency. Longevity of the pump might also be affected.
As far as warrantee, Premacon might also only honor the pump warrantee if you're using the correct oil as per the manufacturer's recommendations, in my case 22 wt oil. (Specifically for the 944 model....the 956 or 960 might use different oil if the pump is different, and I think it is.)
For those in the USA, RC4WD now sells 15 and 22 weight "anti-wear" hydraulic oils in 16oz containers. Expensive, but better than paying for a 5 gallon drum.
Paul