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Hi Andrew,

I drive a Honda Accord Euro. During the week I do mainly short runs to work, gym and back home. Most of the time its in a fair bit of traffic so i use 91. I travel home to Hamilton on the weekends, on these trips I fill up with 95 as its a 150km straight drive with no traffic. Honda have told me the car is designed to run on 95 but will do alright on 91 with a slight loss of performance. Just last week my mechanic informed me that I should stick with 1 fuel as mixing them and alternating frequently between the fuels can course problems with the engine. Is there any truth to this?

Mixing them will not cause any problems. This engine is designed to be run on 91, but will perform marginally better on 95.
We actually used the Accord Euro to do some testing a couple of years back, and found fuel consumption to be 8.6L/100km on 91 and 8.3L/100km on 95, so from a fuel consumption and cost benefit perspective, it works out very marginally better to run your car on 91 as the fuel consumption difference is roughly 3% and currently 95 costs around 4% more than 91. As you can see, it's a real line ball call.
See this link for a little more info.

I've had several Honda Accords for many years, including a couple of Euro Accords. With every Accord I've started out using 95 but then ended up using 98 octane as the car not only performs significantly better but the fuel consumption is also better. The Honda manual states a minimum of 95 Octane.

The better economy with 98 outweighs the extra cost of the fuel. As an example, with my last Accord, a 2007 model, for the first few months I used 95 including many out of town trips. With 95 I managed to get a city/county consumption of about 9.5 Ltrs/100k but with 98 it came down to 8.5 Ltrs/100k. I know these figures are accurate as I have always kept a running logbook (spreadsheet with accurate calculations) and these figures are averages over many tank fills.

Suggest you give your next out of town trip a full tank of 98,