Also, recommend oil weights have tended to drop over the years as manufacturers have chased fractions of an MPG to raise their fleet averages. The same thing with "lifetime fill" auto-boxes which mean that the manufacturer can shave off a few litres of oil usage over the projected lifetime of the vehicle.

I once picked up a V12 Jag that I had to drive back to the UK from the south of France. Before I picked it up it had been taken in for a service to make sure it would make the journey. Within 50 miles of setting off the oil pressure was down to 15-20 psi running, dropping to near zero at idle. The oil on the dipstick looked awfully thin so I called the garage that had done the service and asked them what they had put in it, Oh, 5W-30 the same as we put in everything these days. Problem is the V12 engine was designed around the same time as ours and the sticker under the bonnet said to use 20W-50. Found a garage and bought a gallon of 15W-40, the thickest I could find and did an oil change over a drain. Oil pressure back up to 45-50 running and 20 psi at idle. As Morat says, getting the best economy figures they can manage rather than engine longevity is the manufacturers main interest these days so by specifying thinner oil might gain them an extra 0.5 mpg. Do they care if the engine is worn out after 100,000 miles? No of course they don't, they want you to buy a newer car.