Hmmm, a 7-bolt 4G64 with major oil leaks at highway speeds but not around
town, and AC seems to make it worse. Interesting problem, with several
possibilities.

Since the oil pan has been sealed, its unlikely to be that. When AC is on,
there's more belt load on the main crank pulley. These pulleys are a two
piece design with a rubber insert, and the pulley tends to separate with
age. All this usually does is make belts squeal and causes rattles, not oil
leaks. If the pulley itself is loose on the crank, the front oil seal (in
the oil pump housing) could be compromised.

Check the oil pressure switch on the oil pump housing. I had a failure with
oil leaking out the switch itself, but only above 3700 rpm. Around town I
rarely hit 3400), but on the highway, I cruise at 4000 or lots more when
passing, and the oil blew out everywhere in less than 100 km. Look for oil
dripping out the terminal when the engine is running. If the alternator is
covered in oil, start with the switch.

The oil filter casting is secured with 4 bolts, and a gasket to the oil pump
housing. Unusual for leaks here, but not impossible.

There's other oil seals behind the balance shafts, but these rarely leak.
Next to the oil pump sprocket there's a screw-in cover that hides the
balance shaft bolt. There's SUPPOSED to be an O-ring behind this.

If either of the balance shaft nuts are loose, the shaft could move and oil
will leak past the seal. You would normally have rattling noises if this was
the case.

The cam has a seal that sometimes leaks, but this is almost always an all or
nothing situation.

There's a gasket that seals the oil pump housing to the block, and there's
an oil gallery up near the head that sometimes blows out, but again, its an
all or nothing situation.

Lastly, the 7-bolt engines are (at least in the 2G turbos) notorious for
crankwalk, but its very rare in the 4G64, and I doubt this is the problem.

What you may have to do is get the front covers off, and check where the oil
is really coming from.