I am always using FULL synthetic, like Castrol GTX 5w40 or mobil 1, and i see my cylinder head no sludge inside, also alluminium have this gold color, not brown, or black.
Lidl 15w40 simple oil, 5 liter, is very good for a cheap oil . 11 e / 5 lt.
lidl 10w40 semi-synth is also cheap, like 16 e / 5lt, and i used it.
But very long distance from part-synth to full-synth.
we cannot compare these.
Prefer FULL always.
Car parts shops has ldl quality oil at 2-3 times more price, at least.
About the thickness, for your cold climate, why never 5w40?
this motor has a tension to block hydraulics valve passing with 15w, or 20w
so why should i use thick oil..?
for our motors i suggest use 5w40 or 0w50