Interesting question. I never liked this brand, they surf on the "made in Germany" but the products look quite ordinary.
Do you have an oil pressure gauge? 4G15 are relics from another century (lol), 5W30 is supposedly ok, especially with a HTHS of 3,5 and more.
I personnaly prefer keeping a little margin for fuel dilution and shearing, and 0W40 seem to be a fine go-to grade formy engines.