There are several products that stop oil leaks by causing the gaskets to swell-up. Check with your local auto parts store. As far as I know these won't hurt your engine...maybe someone knows more about these. I have used them before to stop oil leaks with good results. Granted, it is better to replace the valve cover gaskets...not a big job but it is critical that equal pressure be applied to each bolt to prevent damage to the gasket. Over driving them will cut the gasket...hence the need for a torque-wrench, which are relatively cheap and easy to use. Anyone with a service manual can give you the correct pressure setting to use if you decide to do it yourself. Good luck!
Jeff