I'm in Canada. Sold Amsoil as part time business for 20 years. Every inch of cars I've owned since 1976 have been syn lubricated. Have had Fords, Toyotas, GMC and Kia. Never once a warranty issue. A customer did with an after market warranty due to contractual clause agreeing to follow manufacturer's SUGGESTED intervals. He needed a rear wiper motor! Got it sorted for him with warranty company. Generally, if comes to an issue, dealer/mfg must prove a direct causal relationship between the fault and the owner's maintenance procedures. I forced my then Kia dealer to take sign down (in service reception) stating a warranty is voided by not following recommendations in owner's manual. A threat in writing to mfg and dealer to pursue in a Small Claims Court works (from experience) if dealer wants to be an ass.