There is no such thing as a "sealed" oil system on an "open" sump engine. New Mercedes come close with no dipstick. Some ventilation is required due to varying AIR pressures in the sump (downstroke, volatile substances,(ie. gasoline, argh!!), blowby, etc. PCVs do this with positive case ventilation; in the old days the oil filler cap was called a "breather cap" (breathed right into the smog
). The oil filler cap seal is to keep your engine clean; nothing to do with OIL pressure; it does seal AIR to let other emission control devices deal with that. Never saw a car that sooner or later didn't have some seepage from an oil filler cap (no matter how small an amt.).