My 30+ years old BUKH DV10 engine has started to leak oil. It seems that place of leak is "rear outlet shaft seal". Many posts advice to clean vacuum valve/breather valve, so excess pressure is not built inside engine.

I have already located vacuum valve. It seems that DV10 vacuum valve is secured under on of nuts that tighten cylinder head in place (well, how dumb is that). Is it safe to open one of cylinder head nuts and then re-tighten that one nut after cleaning vacuum valve?