We have a Mitsubishi Space wagon, an import, from about 1998.
It uses Synthetic Motor Oil (on the engibne it actualy says "Use Only Shell Helix Untra Fully Synthetic Motor Oil"
I couldn't find any Shell Helix Ultra, but I used another fully synthetic oil.
Howver, i am forever topping up as it always seems to run away over a period.
My mechanic tells me not to use fully synthetic but half synthetic as it is better for my car, and 'sticks' to the engine as opposed to losing oil quicker.
I trust my mechanic implicitly, but i just want a second opinion on this, please.
I don't know about your Mitsubishi, but my own car requries synthetic, and the manual says it is OK to top up with normal oil, up to 10% of the original synthetic.
So if you're doing 50:50, that'd equate to 20% (I think!).