I agree with your comments on warranty.
Please also understand what I am trying to convey.
In owners manual supplied with Toyota Corolla xli, Toyota is recommending API SN 5W-30 oil as preferred oil grade for its cars. There is no mention that the customer "has to" buy/purchase or use Indus branded oil.
The 3S dealers insist to purchase SM or lower graded oils with 20W-50 viscosity. Only oil with API SN rating is Taglon but even this one is 5W-50 not the recommended 5W-30.
Both Taglon and Petron are local brands supplied by Indus. (I am sure they both have no match or relation to the Toyota oils which are marketed globally.)
And then Indus 3S dealers threaten customers with voiding warranty, if they don't buy third class oil whose viscosity grades even does not match with recommended oil grades in the owners manual.
I am sure Toyota Japan will not allow Indus to modify the owners manual so that they can sell Indus branded engine oil. And you are right about that.
Threatening to void warranty in this way has a certain maxim in legal terminology as well and it is "No one shall be permitted to profit by his own fraud, or to take advantage of his own wrong, or to found any claim upon his own iniquity, or to acquire property by his own crime."
So first Indus makes cars which fail (or people think these will fail) early due to poor workmanship and then by creating this fear in customers minds, they sell their own branded oil.
Engine is one thing. But parts like shock absorbers, wind screen, body panels, cooling system, gear boxes, air conditioning etc. have nothing to do with engine oil. Just by using engine oil which is not Indus branded, how come warranty for whole vehicle is void?
I am not against buying OEM parts. I am just saying there are better options in engine oils than Indus supplied oils which are sold by 3S dealers. Toyota globally sells oils branded by itself and those are very good quality oils. An example is the black tin packed oil which was available with 3S dealers around 6 ~ 7 years ago. That was excellent oil better than any aftermarket oil.