Spectrum uses the interfaces and ifMibs to compute utilization, error and discard rates. These are computed internally in the code and cannot be changed by the user.
If you wish to use a different mib, you will can create a custom SpectroWatch. You may need to import the mib into Spectrum if it is not already available.
A word of caution, SpectroWatches can cause a performance issue. Especially on interface/port models. If you create a SpectroWatch and activate it on all interface/port models in the database, it could cause a performance issue.
Joe