just looked at three of the same cars today.?

It's common on a lot of vehicles. You unbolt the valve cover, clean the area, and replace with the new seal, be careful not to pinch or let it roll on itself. Torque it down snug, clean the area, drive it, check it in a couple of days for oil leaking and torque it down a bit more where needed. Some cars also require RTV, you'll have to check with a parts store.

I think any stamped steel valve cover is subject to leaking, if you get a bit ham-fisted and overtorque the bolts through the gasket. I've done the "tighten gently, then see if it leaks" thing. it works, after a fashion.

If you're up to it, you can fab yourself a large "washer" to go over the valve covers and give you some thickness right where you need it- along the head. The next option is to get cast valve covers. The cast ones have an advantage that they frequently have cooling fins, though I don't know how effective those are.

I'd use plenty of gasket sealer on the valve cover, the gasket (both sides) and the head. If I didn't like the gasket material, I'd make my own gaskets, even if that takes some extra effort. I'd like to see at least 1mm of gasket material below the level of the valve cover edge so when you tighten the bolts to the head, you can eyeball how much the gasket material is compressed, not distorted.

Take care!

I Googled "1999 Toyota leak" and came with 114,000 hits. Of course many are duplicates, but that's probably near the number of those cars that were sold in the U.S.

Seems to be a common problem in those car.

it is sum what common for toyota's to have that problem