A. Are you facing any kind of missing?
B. Is every spark plug working fine?
C. How is the performance on petrol?
D. Do you feel any decrease in pick up?
E. Any kind of oil consumption cold/hot engine?
c=on petrol pick is also not good
d=yes, there is no pick
e=oil is getting darker and reducing also, but spark plugs are clear
backfire pipe is producing more air then normal, and some oil is also seen on air cleaner where pipe ends.
Keeping in view the above stated symptoms, I recommend to perform the compression test. There might be some issue with the head/ valve leakage.
please refer this too