Why are newer cars using 20w-5 oil , instead of 30w-5 oil?

Question

Why are newer cars using 20w-5 oil , instead of 30w-5 oil?

Answer 1

The Society of Automotive Engineers, usually abbreviated as SAE, has established a numerical code system for grading motor oils according to their kinematic viscosity. SAE viscosity gradings include the following: 0, 5, 10, 15, 20, 25, 30, 40, 50 or 60. Some of the numbers can be suffixed with the letter W, designating their "winter" or cold-start viscocity, at lower temperature.

Viscocity is graded by measuring the time it takes for a standard amount of oil to flow through a standard o*****e, at standard temperature. The longer it takes, the higher the viscocity, and thus higher SAE code.

Many new vehicles are marked to use 5W-20 oil (Honda, Ford, and more recently Toyota) which is not much thinner than a 30W oil. Nay-sayers of 20W oil's ability to protect engines should note that typically, 30W oils shear down into the 20W range anyway. Most engine wear is during start-up and warm-up period, where the thinner 20W oil's flow is desirable. Overall, lab test results of the wear metals contained in used oil samples show low or lower wear with 20W than 30 in applications it is specified for. Some ultra fuel efficient and hybrid vehicles are marked to use 0W-20 oil. For some selective mechanical problems with engines, using a more viscous oil can ameliorate the symptoms, i.e. changing from 5W-20 to 20W-50 may eliminate a knocking noise from the engine but doesn't solve the problem, just "masks" it.

So in short, thinner oils cause less stress on the engine.  Todays engines are very precisely made and can use thinner oils.