Q. What is the engine oil grade for Royal Enfield Interceptor 650?

It doesn't matter what number is but it's important to understand the main grade which is API and JASO secondly when you say 20w50 and 10w50 then it's a matter of temperature withstand capacity. Main thing in any engine oil when you switch then when you are using mineral then you can jump to semi synth or fully synth oil but you can't down grade or you should not down grade from fully to semi or from semi to mineral. Semi and fully is with lots of chemicals and specifically made inside the lab. Mineral oil has chemicals but very less and amount to is less. So switching to 10/50 is no issue or switching to any other grade also not an issue provided you have to understand your location. Since those numbers are important when your location is colder side or hotter side. You can switch to 10/50 but make sure if early oil is semi or fully then you have to use the same type. All though company recommend 10/50 but that's ok for Europe and North America or Canada since weather is always colder side but for India you can go up to 20/50 right from 10/50 or 15/50 or 20/50. In semi and fully synth chemical formation on engine spares are like film so when you pour mineral then that oil has basic chemicals which can't properly lubricate internal spares. So you may feel. Some roughness. There will be no filming of Mineral oil on any internal spares on which semi or fully synth oil filming is there. It's always best to use HIGHER API and JASO.. I hope this sort out your query. Thanks

Normally it should be changed at 10000 Kms , but depends upon your usage of Bike, if you feel around 7000 or 8000 kms bike engine has more vibration or making more noise and gear shifting is change than the viscosity of oil can be the reason too. So depends upon your riding method. But RE suggests for Interceptor/GT 650 oil change at 10000 Kms with 10w50 Full Synthetic Oil

10w50 is the recommended Oil (Full Synthetic), as you have already used 20w50, I would suggest change it to company recommended standards cause it will/may harm/void your bike Warranty also.