The Ruger Old Army maual says you can use BP from 4Fg-1Fg....here is the excerpt....
AMMUNITION
The “Old Army” is a muzzle loading, percussion revolver intended solely for use with Black Powder. It should never, under any circumstances, be loaded with any type of smokeless powder as the result could be damage to the revolver and injury to the shooter or bystanders. Black Powder is usually classified by powder grain size, with “Fg” being the largest granulation and “FFFFg” being the smallest granulation normally available. Any granulation within these gradations could be used in the “Old Army”; normally, however, “FFFg” is the preferred grain size. Replica black powder such as Pyrodex may be used, so long as suggested loading data is strictly adhered to.