Low Oil Pressue in 2004 Yukon XL Denali with 72000 miles

[DenaliDB]

My 2004 Yukon XL Denali recently had the hoses to the oil cooler lines replaced and the oil was changed. The mechanic mentioned there was metal on the drain plug and showed me the old oil which appeared very metalic. He said he re-filled with 10w30 instead of the factory recommended 5w30 to compensate for wear.

So a few days later, my wife is driving it and the oil pressure drops below 10 (assuming guage indicates 10 PSI). The oil warning light comes on and it starts to ding. I took the car for a test drive the next morning. Oil pressure cold started about 20 and decreased as the engine warmed up. I parked it back in the driveway before the pressure dropped below 10.

The vehicle has about 72000 miles on it and the oil has been changed every 3-5000 miles.

I should mention also that the reason the car was with this mechanic was that the front differential fluid "magically" disappeared after an oil change at the local Jiffy Lube requiring the differential to be replaced.

While the mechanic was replacing the differential, he recommended replacing the oil cooler lines which were leaking at the metal to rubber union.

I find it hard to accept a serious internal engine failure with only 72000 miles on this 6.0L engine. Any ideas? I don't want to take it back to this mechanic, or any other until I have had a chance to do some troubleshooting and to get eduacted.

Thanks to all for your expert advice.