Of course you’re right. It’s PCV gunk. The fuel that gets into the oil evaporates at running temperature and delivers PEA to the back of the intake valves! Geniuses I tell you!
The Pennzoil Ultra Platinum 5W30 tends towards the low side of the viscosity index so you shouldn’t notice any mileage penalty. I think it will be fine year round in So Cal. I’m running it in my F150
I would run a bottle of RedLine or Techron Heavy Duty injector cleaner since dirty injectors...