I have a Kubota and letting idle is bsd for it. Mine is set up with the tier four system to burn off exhaust, letting it idle will throw it into a combustion mode. I have to run mine at or over 2000 rpms
You shouldn't use 8/3 for a 30 amp breaker, the coded size for 30 amp is # 10 wire, any size larger than that with a 30 amp breaker could cause a problem. It would take longer for a breaker to trip