Like Phil, I steer clear of additives. Adding a solvent like seafoam to oil can't possibly lubricate the engine properly.
In proper concentration, as recommended, it will not change the
lubrication properties of the oil to any noticeable extetent.
However, 5 or 6 oz, as stated before would likely cause a noticeable
change in the viscosity of the oil. I doubt very seriously it would cause
any major problems, but it's always better to use the recommended amounts.
When dealing with such a small amount of oil (~2 qts), 5 or 6 oz is a good
percentage of the total fluid.
SeaFoam and the other additives are nothing more than some
toluene and mineral spirits that act as cleansing agents.
They are not really intended to be put in an engine with fresh oil and run
the full oil change.
The stuff is intended to be used for in a couple hundred mile runs....like
adding it a couple of hundred miles before an oil change to help
clean up the engine just before a drain.
It's not really even recommended to leave that stuff in the oil
during a cool down period. It should be put in, ran, and drained while hot.
For wet clutch applications, such as our bikes, I would substitute some
Dextron III ATF for the SeaFoam in the crankcase.
Dextron III ATF is designed to 'wet clutch' applications, such as in automatic transmissions.
ATF is very high in detergent and will help cleanse the engine just as well,
if not better than SeaFoam, and will definitely not harm the clutch
or other internals.
And just as a side note, if the oil is changed on a regular basis and
the proper maintenance is followed, no crankcase additives are needed,
as DynoBobSlick and Phil stated.