The lower the idle the more rattle ya get is normal , Idle should be around 1800 rpms any lower than that and it will rattle .
I have been using amsoill 100:1 for years mixed at 100:1 in all my engines( airplane and utility ) with no problems at all .
Not preaching amsoil just sharing experience. Use it with confidence (or any mfg oil for that matter) as long as you use it as they describe (mix it in the proper ratio)