Oil cap with drill hole in it?

Leyland Roadrunner (about 20 years old) has an oil cap with a hole drilled in it to reduce back pressure. One guy says the engine is likely to die, another says its a common way to reduce pressure if the breather pipe is blocked and an easier way to fix so dont be worried by it. First guy says the breather pipe is 1" in diameter so isnt easy to block although the oil in the lorry is currently like treacle. The dilema is to buy or not to buy- anyone out there got a crystal ball or some knowledgable advice?