[s-cars] oil consumption

[stefan13 at att.net]

During the recent timing belt install, my mechanic changed the oil and put Pentosin 5w-40 in the car.   I previously used only Mobil1 10w-40 (green cap), which the car consumed at a rate of about 1 qt per 3,500miles.  with the 5w-40, the car uses absolutely no oil, nada, zilch.  Somehow, this seems counterinuitive, and I am wondering if this is due to the viscosity or a manufacturing difference.

As always, thanks for any input.

Stefan