The XT needs Premium fuel. E10 is gas watered down with 10% ethanol and can be made with any octane rating. E10 is good in cold weather but has less energy so you get reduced fuel economy.

 

E-10 has less energy than any 100% gasoline, but the alcohol in it raises octane as it increases the fuel's resistance to spontaneous combustion due to heat and compression. Alcohol in fuels used in some racing allows the engine builder to use much higher compression ratios ... tighter squeeze ... like pulling a bow further back before letting an arrow fly.

Turbo chargers push the air / fuel mix into the cylinder at high rates than simple atmospheric pressure can do in the presence of vacuum.

Water has a really huge octane rating ... we can't measure it even ... it contains little to no energy that we can use in a IC engine because it is not combustable, spontaneous or otherwise. You've heard of water injection with turbo or super charging. Result is very resistant to preignition, but the water adds no power, it just cools, controls the combustion.

High octane gasolines also allow higher compression ratios, but they contain more energy than alcohol per teaspoon full.

I rambled.

 

Thanks for the answers, have always run on premium but was told by dealer when purchased XT 95octane and e10 are on fuel filler so that’s what to put in it ! Had no choice on one fill up during shortage so used e10, seemed to run hotter with better performance ! Not a money thing just want what’s best for the turbo/engine

 

You don't need premium You "CAN" run on regular. I use 89 Octane

 

I use 87 Octane. However, will use 91 when on high altitude trips.

 

Note, almost all gasoline in the USA is "up to" 10% ethanol, in any octane rating. I suspect something different may be the case in the UK. As I've mentioned in other XT comments on this forum, I myself sometimes violate Subaru's recommendation for my car IF I'm not planning "hot rod" driving, AND it's cool weather, AND it's humid/rainy, and especially if I'm at high altitude. All of these factors reduce octane requirements, but I only do it as a "at my own risk" thing. Subaru's requirements sensibly simplify requirements to something that can be applied universally (but at added cost).

 

Oh ! Confused again now as some say my xt needs premium others saying regular !

 

@Richard w You're talking about a UK XT, with a map for UK fuels. American fuel standards are very different.

You can use 95 E10 in your Forester if you can't find any premium on occassion but I wouldn't make a habit of it. Your engine is designed for 97+RON premium and will perform much better with it. Most Subarus do best on Shell's V-Power or Tesco Momentum that are typically 99RON.

Each time you use 95, expect the mpg to drop like a brick and could take up to 3 tanks of 99RON to get the performance and MPG back to how it was before the 95.

 

That is great info many thanks, as said always use 99 octane. Why don’t Subaru state this on fuel cap instead of 95 octane

 

@Richard w My son had his XT (actually an "S") tuned to run on 95RON due to lack of 98RON in his area. He said it loses a little bit of power but not much.

FYI
The standards are:
Research Octane Number (RON) is used in Europe and most of the world but not USA
Motor Octane Number (MON)

Then there is Pump Octane Number (PON) in the USA - is the average between RON and MON
For example 98RON is 93PON

I'm happy we use RON rating down-under!

 

@Richard w My son had his XT (actually an "S") tuned to run on 95RON due to lack of 98RON in his area. He said it loses a little bit of power but not much.

FYI
The standards are:
Research Octane Number (RON) is used in Europe and most of the world but not USA
Motor Octane Number (MON)

Then there is Pump Octane Number (PON) in the USA - is the average between RON and MON
For example 98RON is 93PON

I'm happy we use RON rating down-under!

I thought the Yanks used Anti Knock Index (AKI) ?

 

The engine management will automatically adjust the spark and fuel curves for the octane available. It cannot sense what's in the tank, but it can from the knock sensor input. Lower octane will obviously knock a bit sooner, so the sensor hears that, and tones down the tune so it doesn't knock. This is all usually well before you'll hear it.

That's why performance and possibly fuel economy may take a hit with lower octane. It depends on how it is being driven. If you drive fairly conservatively, you'll probably not see much change.

Consider that the N/A direct injection engine runs on lower octane fuel with 12:1 compression says a lot about how well spark knock is controlled. 15 years ago, it wouldn't have happened. ~10:1 yes, but I doubt 12:1. Given that is N/A, it gives the boosted engine a decent window before the boost effectively raises the compression ratio above that.

 

My friend bought a used XT and the idiot salesman told him to run regular. After his first fill up a check engine light lit up. He used Premium on the next tank (recommended by a tech at the same dealership) and the light turned off. The first thing the tech said was "You used Premium fuel correct?"

 

I just assume that I would be driving with more boost at higher altitude so I better be safe by using the recommended higher octane. To be honest, I never had any problem using 87 octane during short trips over mountain passes, and I go skiing a lot.

 

Seems that the recent turbos tolerate 87 Octane, but reduced max. hp.