MerlintheMad
10000 Posts Club!
Don't worry about octane. Anything 87 and up is good. If you mixed 87 with 91, you'd just end up with "midgrade".So what should i do to convert my 87 to 91? Just burn off the rest and put in 91 directly next fuel tank? And what would synthetic lifetime be?

I've never used synthetic in my vehicles. But I've read what those who swear by it say; and I've read assertions up to twelve years, and even the assertion that if your oil keeps coming out clear to amber on the dipstick it can last indefinitely; just keep it topped up. (Nobody with a Stinger has said that.

The only problem with going longer than recommended is that warranty has to follow the manual and service periods. And Kia has said cut the recommended 6K miles per oil/filter change to 3K for "severe" driving conditions; and the manual lists what those are. Utah is a "severe" climate because in winter our roads are salted down. But that's only for a couple to three months max. The rest of the year we are paradisiacal.

If the dealer says yes, then we are covered. If you want to know if synthetic changes what the manual and service periods should be (i.e. increases the miles/months between), the only "authority" will be the Kia service center.