This isn't a very common leak. I would suggest trying to tighten the cap first. If it seems tight enough, the seal is actually aluminum, not copper. When you remove the cap, there will be a strong spring that will release tension, be careful and you won't have a problem, it will be equally challenging to reinstall the cap though. The spring is important to hold pre-tension on the chain tensioner on start-up, before oil pressure is built up. The end of the tightener has a one-way mechanism, in case the oil pressure is lost to the tensioner or the pre-tension spring breaks, so because of this one-way action, you don't need to worry about taking the cap off too much, it should be just fine.