On what basis? Experience for one, and common sense for another. The manufacturers admitted even then the low viscosity oil was STRICTLY for C.A.F.E. reasons. Every 10th of a MPG they could gain in fuel mileage was a HUGE bonus. In the 10 years I spent as service manager at a very successful Toyota dealership, we NEVER had an engine failure of any sort on vehicles we serviced regularly. Not a camshaft, timing chain, or engine bearing. NOT A ONE.We serviced over 600 vehicles on a regular basis and had about 1000 on "active" status on the cards. Many went well over
300,000Km. Yes, we had the occaisional head gasket issue, particularly on the "T" and "M" engines. No oil under the sun could prevent(or cause) them. Yet I had many vehicles in for engine work that were sold and serviced by the other dealers in the area who slavishly followed the "book". "M" series camshafts, timing chains and tensioners," R" series timing chains and the odd oil pump, and the occaisional bottom end. "S" series timing chains were just starting to require replacement when I left The service rep asked me why our warranty instance was so much lower than the other dealers and I told them what we were doing. He said they'd get skinned alive for recommending it, but it was undoubtably the best practice for engine longevity. The oil was changed 4 times a year - 20W50 went in from March 1, more or less, 'till roughly September 1, and we used 10W40 the rest of the year. My brother had the same experience at a Ford dealership with Pinto, Mustang, Ranger and Aerostar 2300s. With 20W50 summer oil they never had a camshaft failure -nor did the dealers in the south using 20W50. With the recommended oil camshaft failures were not uncommon and the factory reps were telling the dealers they could not officially say it- but running 20W50 in the summer, particularly south of the Mason Dixon, was really a good idea on those engines.
The lifter problems that plagued the 230 and 250 inch Chevies on 10W30 were significantly less common when the heavier oil was used, as were the camshaft failures on Chevy 307s. The rocker ball wear on Ford 351 C and M engines was a lot less common with thicker summer oil as well.. And that was back when both oils still had Zinc and Phosphorous EP additives.