The millers is probably less volatile. Under heat it is staying as oil instead of ending up as vapour and being pushed out through the breather system.
I would be tempted to try something like Fuchs Titan Race pro S Fuly synthetic.
the problem with a 5W-50 is there there is a massive gap between the cold viscosity rating and the hot. The oil will be a thin oil, with added viscosity index improvers to make it thin less when hot (so it still behaves like a 50) the problem is that these additives break down over time, and you end up with a thinner oil when it is hot.
The bigger the viscosity gap, the more VI improvers and the more it happens.
Giulietta 2.0JTDm (170), BMW 335i Cabrio