3.9L recommended fuel and tank capacity?

[SFD90]

I always put 91 octane in my D90, but I was curious if this was necessary since I don't know for sure? Also, I've put up to 13.5 gallons in it, but what's the max?

Thanks

[rover4x4]

around 15 i think

[Eric Siepmann]

The owners manual for 97 suggests 93.

EWS

[Mike Hippert]

I'v put I think 14.6 gallons in one time, I always run 91 or higher in it, the owners manual says to put in 91 or higher and just as Phillip said the tank is 15 gallons, not quite big enough.

[Art Vigil]

The tank is 15.6 US gallons.
You don't necessarily need to run the highest octane you can buy.
Higher octane gasoline helps to keep your engine from pinging which can cause serious damage in a short amount of time.
Pinging usually only occurs when your engine is very hot and under full throttle.
The hotter your engine is, the more likely it is to ping.
I run higher octane gas in mine only because I have a supercharger which can cause the engine to operate at a higher temp than normal. I also use an octane boost before long road trips since I know the engine will be under a greater load.
In my last D90 ('95) I only ran the lowest octane gas available and never had any problems in 80K miles.
The octane level only prevents pinging, if you don't need it, spend your $ somewhere else. You can experiment with lower octane and depending on your vehicle & driving habits find a level that works for you. Lots of people think they're "treating" their engine when they put in high octane gas, I don't believe that to be the case, although I suppose peace of mind counts for something.
My .02

[scoloco]

At higher altitudes you can sometimes get away with running lower octane fuel. But if you are at one of the coasts and near sealevel, you probably want to consider running super to prevent pinging.