Exhaust system and Oil pressure light problems

25 yr old truck=low oil viscosity=high revs. I don’t expect good results.

Is 0w-16 more expensive than other full synthetic oil? My Highlander calls for 0W-20.

If the dealer buys 0W20 in 55 gallon drums and dispenses directly from that barrel, it should be a lot less expensive than if they have to buy 0W16 oil in quart cans or 5 quart bottles. The 0W16 would also take up shelf space in this scenario, also adding to the cost.

Maybe a shop can post bulk oil costs?