Ive got a Suzuki AerioSX with a 100k warranty, I goto the dealer to get my oil changed for 10 bucks and this is a perfect time for them to address my warranty concerns and I like the Suzuki OEM oil filter so thats why I dont change my own oil...but the first time I went there(1500 miles), they changed my oil, the car slowed down and I got no where near the claimed MPG...I just figured the car was breaking in...I go back for another oil change, and for some dumb reason after they finished I asked "what kinda oil are you using?" Realize this is primarily a Jeep dealer, and thier service department and employee parking reflect this...the guy says to me, "the samething we put in all of our vehicles...15w40, why? Is there a problem with that?" I say,"yeah theres a problem, the friggin oil cap tells you what to put in...whats the problem with that?!?!"
From that point I switched to Mobil 1 syn 5w30 and bring my own oil...I also asked if they had a technician that drove a Suzuki, they said yes...and I said "thats the only person I want working on my car!"...and now I never leave my car while having a service provided. There have been several occassions since, that if I had left, my warranty concern wouldnt have been fixed, with the claim "it didnt do it", "we didnt hear it" or "its fine"...I find now that the mechanic asks questions, and because Im there to answer the problems get fixed, instead of just being parked with nothing done/fixed.