Nope, never used Mobil1 in the Belta. Shell Helix Ultra is fine, I have been using it for the past 4-5 years. I replace it at every 4000-4500Kms. This one was 5W-30 instead of the previous 5W-40 Shell use to have.
I did mention somewhere that I would start using Mobil1, will hopefully use it next time.