If it's got bore score/knackered rings/valve seals and passing the rings thinner oil will be worse......sounds like Magnatec is gnats **** thin (hence them recommending a thicker viscosity).
It's happening when it's getting hot because the oil runs thinner when hot.
Read below.
Bad valve seals will show themselves during prolonged idling at stop signs or traffic lights in congested city conditions. When the vehicle sits at idle for prolonged periods, high levels of vacuum at the intake manifold result because the throttle valve remains closed. The high vacuum attracts oil in the heads to congregate around the valve stems. Upon acceleration, the oil gets sucked past the eroding seal and down through the valve guide, where it burns in the exhaust. Huge clouds of blue-white smoke exit the tailpipe after each acceleration from a stop. The burning smoke will disappear during cruising or highway speed.
Sounds like another one looking for an engine happening more and more this malarkey. That engine we took out (and by we I mean the royal we......i.e Chris the mechanic at my place while I watched and made tea) last week had majorly shat itself but it's very hard to diagnose why.