Below is an abstract from an old SAE paper. It is fairly representative of several other papers. Given the observations below and elsewhere, why would A3 always be better than A1?
Influence of Engine Oil Viscosity on Piston Ring and Cam Face Wear
Document Number: 932782
Date Published: October 1993
Author(s):
Toshihide Ohmori - Toyota Central R&D Labs., Inc.
Mamoru Tohyama - Toyota Central R&D Labs., Inc.
Masago Yamamoto - Toyota Central R&D Labs., Inc.
Kenyu Akiyama - Toyota Motor Corp.
Kazuyuoshi Tasaka - Toyota Motor Corp.
Tomio Yoshihara - Lubrizol Japan Ltd.
Abstract:
The influence of engine oil viscosity on the wear of piston rings and cam faces has been investigated by fired engine tests using a radioisotope (RI) tracer technique. High-temperature and high-shear-rate (HTHS; 150\mDC, 10\u6s\u-\u1) viscosities of the experimental oils prepared are 2.2, 2.4, 2.6 and 3.1 mPa\mks. At an oil temperature of 90\mDC, the wear of piston rings and cam faces did not increase, even if the HTHS viscosity was lowered down to 2.2 mPa\mks. However, both piston rings and cam faces exhibited an increase in wear below 2.4 mPa\mks at 130\mDC. It was also recognized that valve train wear did not significantly increase with reducing viscosity in the motored engine tests at a temperature of 50\mDC. From these test results, it was suggested that the oil with the HTHS viscosity of 2.6 mPa\mks sufficiently demonstrates the anti-wear performance equivalent to that with around 3.0 mPa\mks for application to piston rings and cam faces.
