Just done some online research on what oil companies recommend for my 1.6 MK1 and came up with some interesting options. All these were using the oil companies own research tools where you enter your car reg and then your make and model of car pops up.
Mobile: Mobile 1 New Life 0w-40 synthetic
BP/Castrol: Castrol Magnetic 10w-40 part synthetic
Have gone with the Valvoline for top purposes as I’m not sure what the garage who sold me the car has put in it and also it looks good on paper: http://www.valvoline.com/pdf/maxlife.pdf
There are no proven reports of Mk1 or Mk2 cars being damaged by any 5W/30 to 10W/40 oils and Mick has had good results from 10W/40, I have also had good results from 5W/30 in that we both have had no lubrication problems.
The MX5 engines are very soft on oil.
What makes the Valvoline you mentioned any better than the other oils.
It looks to me to be a pretty run of the mill oil no better or worse than the others mentioned.
It might well be run of the mill but it was the only oil that mentioned it was designed for high mileage engines and in regard to this I’ve read that because of possible leakage issues with older engines it’s best not to use fully synthetic.