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.
Arthur M Vigil
Former owner of:
'95 RRC - 3.9V8 Rimmer Supercharged!
'72 Series III 88" NADA Turner 2.25 petrol (Lovely Rita)
'95 RRC Brooklands/TWR Edition 3.9V8
'97 D90SW NAS #2426 - 3.9V8 Rimmer Supercharged!
'95 D90ST NAS#2620 - Original owner