1GD is the most modern engine from Toyota. It is very efficient and environmentally friendly and has the capability to clean burn even the dirtiest diesel. I am mentioning this to just clarify that do not compare it with a C or L series engines from Toyota which came in 80's.
Your Fortuner as I know has EGR and also Catalytic Converter as well as a very latest turbo charger. These are sensitive to the engine oil you are using. Using a high ash bearing, high viscosity rated, mineral, high volatility etc. engine oil is not very good for your engine life as well as economy.
Even Pakistan Toyota as per Owners manual has recommended a 5W-30 engine oil with CF, CF-4 or ACEA A3-B3/B4 rated oil.
Shell Helix Blue Bottle oil has exactly same specs and it is economical too.
I am using Mobil 1 0W-40 oil in my 1GD Fortuner with oil change interval of 5,000kms.
In hilly areas like if you have been driving upto Babusar Top or Khunjerab Pass most of the time than a slightly shorter change interval may be 4,000 kms is recommended. It is because a diesel engine produces more soot at these altitudes.