1990 Toyota Corolla engine oil leak

I've got a 1990 Toyota Corolla DLX with a 1.6L 4 cylinder 4A-FE engine.
About a month ago I noticed knocking sounds out of the engine and I decided I'd have all my oils changed at a oil change station.
It turned out there wasn't any oil in my engine!
The oil got filled and ever since I have to throw in a quart of oil about every 400 miles. (I drive 100 miles a day)

As for symptoms.
I've noticed a white "smoke" coming from my engine sometimes after a long drive and I found that this is caused by oil dripping on my exhaust pipe during the drive.
Also the oil pan seems to be very wet with oil and every now and then (not always) my car leaves a small puddle of oil near the front right tire.
By now the entire engine is wet with oil so it's difficult to point at the specific point where the engine leaks, I think it might be the oil pan gasket, but like I said, the entire engine is wet in that area so I can't be sure that that's it.
Also when my car idles and I look from underneath the engine I notice some sort of "discharge" from right underneath the exhaust manifold which looks like a little bit of fumes and a little bit of spray.
It's not coming out of the exhaust manifold, but somewhere from the engine block underneath the manifold.

Does anybody have any idea what this could be before I start taking things apart to "shotgun" parts at it?