A seller says a used 6.0L LS just needs heavier oil because the pressure reads low. Should I believe that?

Be skeptical. Thicker oil can mask worn bearings for a while — it can even quiet a rod knock temporarily — but it doesn't fix anything. Low oil pressure on a used engine is a warning sign, and worn rod bearings are a common reason the pump can't hold pressure.

Before you pay:

• Check pressure with a mechanical gauge hot at idle and at higher rpm, not just the dash gauge or sensor.
• Listen for a deep knock from the bottom end that gets louder under load (see lifter tick vs rod knock).
• If the price assumes it's healthy, walk away or price it as a rebuild.

Rod knock almost always means major engine work — the fix is bearings and crank work, not oil viscosity.