For head gasket, check oil and water aren't mixing, Given that the oil looks 'typically black' it may not be that, but a implement compression test will show this, even if it's blown between a piston and an oil way
Years ago my old man had a RRC 3.5 v8, it was forever blowing the V gasket and pushing oil out, change v gasket twice before a friendly landrover guy showed us the blocked 'flame trap breather' on back of rocker cover. No problems since then.