My 05 Passat TDI is leaking oil. I can't find the source, but definitely passenger side, and near the top of the motor. I've replaced the valve cover gasket, tandem pump gasket, coolant sensor o-ring, various o-rings on the turbo hoses. There is what seems to me to be a lot of pressure at the oil cap, as if there is blow-by, but I never checked that before, and maybe it's normal. I've also noticed a drop in mileage recently. It coincides with a switch to studded snows and a covid-related change in use--most driving is local, sometimes not even long enough to warm up fully in winter. But still, I'm down around 28 mpg, and barely cracked 30 on a recent 50-mi drive on the interstate. Thoughts?