I have a 1996 Chevy Sliverado 5.7 vortec engine timing cover gasket issue, I think. The gasket has been replaced by a reputable mechanic. The oil pan gasket as well as the right valve cover gasket has been replaced.
I have carefully cleaned the entire area and blocked both sides of the timing cover with paper towels to ensure that no oil is dripping down the sides. After letting the truck sit all weekend, oil is collecting at the bottom of the timing cover. I can only conclude that the gasket was damaged or improperly installed. I hesitate to ask my mechanic to do his job over again but. For over $500 to do the timing cover gasket and oil pan gasket, I don't think I'm asking too much.
I'm an electrician and can't have my vehicle leaving oil spots on customer's driveways. All told, I've spent over $600 chasing this leak.
If you mis-wired a light switch would you be angry if the customer called you back to fix it? Would you try to charge them for correcting your mistake? I ran into this too as a tv repairman. Sometimes you think you repaired an intermittent problem, then find out the problem is still acting up. The only exception with your mechanic would have to do with who diagnosed the leak. If you did and asked your mechanic to replace the timing cover gasket, it would not be his fault if the wrong part got replaced. If he diagnosed the cause, he was either incorrect or, as you suggested, put the new gasket in wrong or it got damaged. In those instances he can be expected to clean the area and re-diagnose the leak at no charge to you. If a different problem is found, you could be expected to pay for the parts, but very often the shop will cover the labor, especially if you are a regular customer. At the very least, give your mechanic the chance to check his work. The last thing he wants to find out is someone else had to come behind him and fix the problem. One tool that works well for finding leaks is a smoke machine. You inject white smoke at 2 psi into the dipstick tube, then watch for where it comes out. That should be pretty easy to spot for a leak as big as you described. The timing chain cover could be cracked or the oil could be running down from the front of the intake manifold.