What is the oil capacity of the 2006 roadstar silverado?

Behind the left side oil cover under the seat. Remove the bolt on the cover and pull it off.

run higher octane or octane booster in fuel

Any oil that is JASO MA certified motorcycle oil. 10w-40 for cold climate. 20w-50 for warm climate/touring. Car oils use different additives. Do not use any oil that says Energy Conserving on the label . It will shorten the life of your wet clutch and transmission gears. If you use Synthetic oils, they will help your bike run cooler and shift easier.

The Yamaha Roadstar has a dry sump oil system. What this means is that the oil is stored in an oil tank to reduce the size of the engine. By storing the oil in the tank (which is under the seat and has the filler cap/dipstick, the engine does not need a sump under the crankshaft to store the oil. In this type of system the oil is pumped back to the tank after making its rounds through the engine. When you shut the bike off the oil will settle in the engine until you run the engine. The procedure to check the oil is to warm up the engine and allow it to idle for approximately 3 minutes. Do not rev up the motor during this time as it will cause the oil level to not read correctly. After shutting off the motor, check the oil level and adjust it to keep it in the proper level on the dipstick.

Contact a Yamaha dealer. Using the incorrect amount, or wrong type of oil could lead to having the forks collapse while riding. It will vary also from model to model, so just because it works on another bike doesn't mean it will work on yours.

should just turn the key to the left unless the cable release is broken

Sounds like jetting is to rich, recheck settings and fuel height settings, need to get the fuel mixture correct as that bike will develop other running issues due to carbon build up in the rear cylinder. check out www.roadstarclinic.com as this is a good source for information on this bike