Oil thins when it warms - or thickens when it cools - depends where you start measuring...in a nutshell, the oil pump should pump enough oil that you develop 10 PSI per 1,000 RPM when the oil is at operating temperature (right about 100 C...). The warning on the dash comes on at 8 PSI (or close to that...way too late...). If you increase the viscosity, the pressure goes up (more resistance to flow). So, you'll have higher pressure cold than hot.
If the oil pressure is low, there are a couple of possibilities: 1. wrong viscosity, 2. failing pump, 3. blocked pick-up and 4. worn bearings.
But going over the description of your history, including frequent oil pressure warnings when hot lead me to think that worn bearings are most likely, because the pick-up was cleaned and the oil replaced. The turbo was the victim of low pressure - and the engine was operated repeatedly with low pressure.
What concerns me about your description was the "hope for the best"...he should have put a known good gauge on the engine (it's easy, pull the pressure sensor and attach there) to verify that the car had good pressure after doing the oil pan...if the o-ring was bad, then that would have shown up right then...and might have saved the turbo...
If the pump was failing, it would show up as low pressure when cold and warm...but since the car only warns you at extreme low pressure (like 8 PSI), the gauge is the only way to determine that...
Get a good oil pressure gauge on this thing and figure out what is going on before you destroy an engine too...
2006 Corolla (kid's car - 132K, Conti Eco Contacts, Castrol Edge 5W30)
2005 MB S600 (74K, M1 0W40)
2002 V70-XC (178K, Nokian WR G2, Pennzoil Ultra 5W30)
2002 V70-T5 (156K, IPD bars, Bilsteins, Extreme Contact DWS, Mobil 1 5W40)
1992 MB 300E (Sportline - 137K, Nokian WR G2, Liqui Moly 5W40)
1990 4-Runner (V6, 4WD, mild lift - 250K, 31" BFG AT KO, Edge 10W30)
1932 Packard Sedan (straight 8, dual sidemount spares, original paint and interior, Shell Rotella 15W40)