davincity

So guys I figured out where my leak was coming from. I have a 1991 LS400, I cleaned the bottom and there was a snail trail from a little black box (timing cover?) above the oil pan gasket. My mechanic took it apart, replaced the crank seal, and he said the valve cover gasket wasn't leaking. Only thing I can think of is the cam seals?
I really need help with this, I had it serviced 3 times, they fixed an O ring, Crank Seal, and oil pan gasket, nothing worked! I'm just thinking what else could possibly be leaking oil. Also replaced the bearings in my drive belt tensioner and it's still squeeking.. is this because oil is leaking onto it? Any help will greatly be appreciated as I don't know cars to well. Thanks guys!