In terms of safety and the health of the engine. 91 Octane is find, unless you are towing or racing the engine , anything that the ECU has been tuned for is ok. :) B)
I do mix the fuel ;) . Tell you what I found regularly.
I found 91 Octane have much worse fuel consumption compare a full tank of 98 <_< . I have to honest <_< and said I have not tried the 95 yet and I never have experience in them. They are the slowest turnover fuel there is in the servo. So I think fresher fuel is better (hence 98 Octane).
I tell you my experience. My wife and I just done 10000km in the car.
1. On city runs (around Kew and no freeway), although it is hard to know since the mixture is not the same everytime:
I get about 16.5L/100km on 91 Octane
but I get as low as 14.9L/100km on 98 Octane, but it is normally in the low 15L.
when I put 10% 98 with the rest 91. I get 15.9L/100km
2. I do long trip twice in this car, that is freeway only:
I get about 10.2L/100km in the 91 Octane.
But I get 8.9L/100km in the 98 Octane. *(then I go to the Great ocean road and it increases to 10.2L/100km, so I dunno)
3. One a time I tow my sports car, full 98 Octane gets 16L/100km.
Back trip uses the V power racing (100 Octane), I get down to the 14sL/100km
I think when the fuel is only 10c per litre more. It is like around 7.5% more of the 91 Octane. I think there is under 10% increase in fuel consumption. I thought it is very marginal. But the mix fuel is only marginally more expensive, but I get 3% better economy.
Go figure. haha.
I was told by my dealer that though:
When towing use 98.
Every 1 of 5 tank use 98.
I think it is logical.
Plus 98 has more power. So when I am driving it.... use 98. When my wife is driving it use 91 haha