We might need to look at some terms here. A dealership cannot really "void" a warranty issued by the OEM or a third party (used vehicles or extended new vehicle coverage), BUT they can deny coverage (for various reasons). Extended warranty company reps can't really "void" anything either, just deny coverages on a case-by-case situation. If the warranty was voided, then NO warranty coverage of other items or related items would remain on the vehicle.
If coverage is denied, by whomever, they need to have a valid, reasonable, and realistic reason for their orientation. On the GM side of things, a service consultant can access the GM warranty records for GM vehicles . . . which can sometimes indicate "flaky" mileages or possibly can indicate some patterns in the type of work that's been warrantied on the vehicle. I do believe that some "reporting" repair shops do report their service records to CARFAX, too, as many dealerships that use CARFAX also do . . . which makes it all "public information" of osrts.
I do know that in the case of GM NorthStar oil consumption issues, in the earlier 2000s, we DID warranty new engines right before the end of the warranty . . . as our shop had done all of the oil changes from when the vehicle was sold new to the then-current owner . . . with documentation in our computer database. If that had been somebody who did their own oil changes with so-so documentation, it could have very well have been different. Be that as it may.
We also charged $10.00 labor for oil changes where the owner brought their own oil/filter . . . gladly and reliably. But there would need to be a higher level of TRUST that they actually used what you brought (especially the oil), but then what would be the advantage (other than cost or use of particular brands)?
Remember, too, that EACH dealer is judged by their deviation from "Zone Average" on their warranty costs/vehicle. If the dealaership has a lower average or is at "zone average", plus having a great working relationship (read "mutual trust") with the OEM's service reps, that particular dealership can usually get things covered OR be more willing to get things covered that another dealership might turn their nose up at . . . which might generate the "Show us your maintenance records . . . " comments.
In many respects, it's now to the dealership's advantage to do warranty work as well as customer pay work. AND get covered what they can get covered! It's called "keeping the shop working" which provides profits to the dealership AND benefits the customer at the same time. To me, any Service Manager, Dealer principle, Service Advisor, etc. that does not recognize that fact AND operating under that orientation needs to get their priorities re-aligned. Especially as many of these people get paid off of what they produce in "work" for the repair shop!
NO need for any power-plays! Just do what you CAN for the customer and get on with things! All of the non-related squabbles over the brand of oil and such just hurt productivity and compensations thereof. We ALL know that the GM OLM can be anywhere from 3000miles to 15K miles, depending upon how the vehicle is driven (from GM's own article on such). Many Euro OEMs are now in the 10K range, with specified motor oils, for which the oil specs were designed around for particular brands of vehicles. Kind of makes our 3K or 5K oil change intervals look a little foolish, sometimes! Especially considering that oils which meet the Euro specs are also used in USA-brands' motors.
In THAT orientation, the dialogues at dealerships about warranty denial due to not following some particular oil change mileage interval (under 10K miles, for example) seem "out of place", UNLESS they can prove otherwise (unusual sludge build-up or an oil filter with "engine color" still on it at 30K miles) to indicate "lack of proper maintenance". In those cases, documentation can be key to the issue at hand.
When a mechanic opens up an engine, they can see how much and where any sludge accumulations might be, just as they can see what type of wear is on the bearings (or absence of wear). From what they've seen, they can usually suspect what the maintenance intervals might have been AND how the vehicle was used. Even if there might be some unusual sludge accumulations, their OEM probably has put some visual images in service publications to use as a guide. Weight of the oil filter can be significant, too.
As for letting somebody else do my oil changes, I would look for the brand of oil I liked and seek out those establishments, then further looking for those I trusted to do the work and do it well. In this day and time, NO tolerance of sloppy work or similar! BUT if this establishment only does oil changes and such, then when you need other work, you'll be looking again. Therefore, it might be better to find a dealership you like and feel comfortable doing business with (even particular service advisors, as mentioned, and patronizing them for everything you need done. But each has their own orientations on these things.
Just some thoughts,
CBODY67