FYI: the various warranties that come with the vehicle (Powertrain, Bumper to Bumper, Extended, etc.) are not entities which live until they expire or are declared "void" by the issuer. They are merely individual sets of guarantees which cover certain related parts and functions for a certain amount of time. If something goes wrong within that amount of time, you can ask that the guarantee be honored and the problem fixed for free, under the respective warranty's declared conditions, and a decision will be made on whether or not to honer the guarantee.
However, if you failed to abide by the declared conditions of a warranty, e.g. use an engine oil other than what is specified to maintain warranty coverage, the request for a free repair under the warranty can be denied (but here's the important part) if the warranty issuer determines the warranty conditions were not followed or violated somehow.
There are 2 very important aspects to that last part:
1) To deny a warranty claim, the manufacturer will need to decide you violated the warranty requirements. NOTE: they can decide without an investigation, and it's up to you to fight it, but that's an edge case I believe you shouldn't worry about.
2) A warranty claim can be submitted, and it is then approved or denied. The original warranty remains in effect, barring something catastrophic has occurred which takes out the whole warrantied system. So subsequent warranty claims can be submitted, and they will be approved or denied. The warranty doesn't just go "poof" in other words, at their discretion.
Now if you throw a rod through the side of the block and take it in for a new engine and tell them you ran oil that was not specified in the owner's manual, you should not expect to get another engine without paying for it.
However, if you run Dollar General 20W-50 the whole life, and it chews up a camshaft, you could get the replacement work done for free, assuming you do NOT mention you were running cheap, too-thick engine oil. If you do tell them, expect to pay for the repairs.
But you should not worry that they will do some sort of CSI investigation into what the oil's original weight was when new. As long as it does not look like you neglected to change the oil regularly, you would be fine running any weight you wanted to.