Thanks all,
Temps here range from 15F to 85F max.
Engine uses one quart per 1000m, and uses standard 15W40 oil (Castrol). Cause of consumption is unclear: no blue smoke, dry plugs, no leakage, max oil pressure. The idea behind 20W50 is that this thicker oil is better contained inside the oil system of a 120K mileage engine, compared to the thinner 15W40.
However, if 20W50 is for sunshine states only, its of no use to me in view of the climate. It will be clear to you, that I don't understand the multigrade system...

What type of (mineral) oil is thicker than 15W40, and has the multigrade viscosity that fits the temp range where I live (The Netherlands)?

Cheers!

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2011 Prius