Series 2a Land Rover? That's amazing! On a side note, I'd love to know more about it soon.
I apologize though, I'm not sure where to find that oil in your city, I've never actually been to Pune, hopefully someone will be able to point you in the right direction!
However, I know that sometimes for older cars when the single-grade oil becomes unavailable, manufacturers put forward a recommended multi-grade oil that is also compatible with your engine.
I found this chart that recommends 20W50 http://www.dingocroft.co.uk/acatalog/Rec_lubes_Ser.htm
But you may also want to shoot Land Rover a quick email to double check.