bro sooner the oil gets black means it has more cleaning agents. you can try shell ultra 5w-40 and it becomes blacker even faster so oil getting black is no issue. now for being watery thin means its a chemical not conventional mineral oil so actually that is better too. thinner the better and if actually a 50 viscosity oil is thin than it means it is for sure a good quality oil.
i would still iterate an oil for 3K Rs with 5K KMS change interval is still better than an oil for 5K Rs and change interval of 10K KMs because changing the oils with lesser milage ensure better lubrication and sureity that filter wont fail. Also 5K KMs interval also ensures warranty which is an added advantage.
also the improvement you are experiencing is more associated with oil change and every time you do it (watsoever the oil is) you will feel the improvement.
I used to pour in additives like ceratic way back in 2005/2007 (american made Whiz brand if i remember is correctly) and its a good addition.