Ah the common OEM / oil manufacturer partnership. I'll offer some insight into this as I'm currently working at the forefront of switching oils at an OEM.
In this example, the engines have been developed on 'Oil A' and in the field for ~ 10years. 'Oil A' is factory fill and recommended in the owners book etc.
Marketing decide on a new partnership opportunity. Tell engineering that 'Oil A' will now be replaced by 'Oil B' and that the change will happen ASAP.
Engineering put the brakes on this immediately and provide marketing with a validation plan and timeframe and explain that changing the oil suddenly like this is like giving somebody a blood transfusion without knowing what blood type they are.
Engineering set about understanding all the characteristics of 'Oil B' and ensure it is comparable at least on paper to 'Oil A'. The most important things are cold and hot viscosity for oil pressure, oil film thickness ( bearing loads, friction models, emissions), low ash content for GPF equipped engines, protection for low speed pre-ignition, thermal conductivity & specific heat capacity, total base number and the additive pack.
Only once these are understood and the risks assessed can physical engine testing begin. This typically begins on a dyno and the engine completes identical tests with 'Oil A' and 'Oil B', comparing parameters such as oil flow rates, heat rejection, oil pressure and friction. The oil is analysed throughout the test to assess the wear metals present, fuel dilution, viscosity and total base number.
This then gives the confidence to run this oil on more test engines and engines in vehicles, which will run different cycles as defined in the validation plan. Constant oil analysis is carried out and when the engines/vehicles have completed their tests the engines are stripped and all the components analysed to ensure there is no abnormal/excessive wear.
Once 'Oil B' is signed off from engineering, it becomes the factory fill oil.
It is an in-depth and expensive process but one that is so important.
As has been mentioned previously, it is usually always the valvetrain that suffers from excess wear. It is a tough environment with more boundary level lubrication than anywhere else in the engine. This is usually where the additive pack is really working hard and one needs to understand the level of protection they're getting from the additive pack, and if it is right for their engine.
Journal bearings are usually most sensitive to oil viscosity and this needs to be properly understood.
So yes, OEMs can and do change oil partnerships but there is a lot of engineering work in the background.
Remember also that OEMs may not necessarily use the 'best' oil available, but it is the oil developed for that engine.
For our 50 year old engines in our Vettes I think we can safely assume that all modern oils are superior to what Chevrolet developed these engines with but as pointed out by somebody else, watch the ZDDP levels as modern oils are reducing this in favour of protecting exhaust after treatment
. This is part of the reason behind increased use of DLC in modern engines.
