Try this simple fix for your oil cap. Remove the rubber gasket and turn it around. After years of being tightened down, the rubber has no doubt an impression in it from the valve cover. For the breather hose, try a new clamp like a heater hose clamp.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member