I run Amsoil full synthetic 5W30 in my GT. I change it once a year but I only put on ~10K miles a year. I'm almost at the 60K mark and the only thing on my list is a transmission service.
Seafoam is good, but keep in mind it's good to replace the plugs shortly after running seafoam through the vac line as any crud in the combustion chamber or around the valves can damage/gum-up the electrode. I also change the oil shortly after as well but with only 60K, I won't do a seafoam application or change the plugs to 80K.
If you drive on flat roads all the time and don't race the engine at all, plugs will last 100K. If you have/use the autostick and use the lower gears often like I do, I wouldn't wait til 100K to change the plugs.
Once I can find a way to remove my throttle body to give it a proper clean, I'll remove my plugs and inspect them, oil the thread and reinstall.
'11 Ram SXT 4x4 quad cab - 5.7L V8; 128,200kms
'09 Malibu 2Lt 3.6L - 140,000kms purchased 1 March 2019
'06 Pontiac G6 GT - 3.5L V6; 162,000kms (rear ended 15 Feb 2019, and given totalled title by insurance)