Matthew,
You almost certainly have aftermarket/budget pistons if they have the lower oil ring groove. As far as I know the original factory ones never had it. The reason for it as best as I can make out is that it is cheaper to produce two single-piece cast oil rings than make a single multi-piece one that actually works.
If you use decent rings then you don't need the lower oil ring. Total Seal gave me this info when I ordered their gapless second ring set for a std bore 2639cc engine:
"our part # is S0885 3.125" 3/32 3/32 3/32 3/16 6cyl $233.96 + shipping need 5-7 day lead time to complete. "
Aside from using good quality rings, it is also important to have the hone pattern done correctly for the rings and to bed them in using appropriate oil "like you stole it".
Couple of other things you might like to consider while in there- have the block deck ground down to the height of the highest piston and if the pistons are significantly different in height at TDC (mine were 20 thou different!) then have the crank index ground so they are all the same height. The reason to bring the pistons up to the top is so that the 'squish' zone in the head can work properly which means you can increase the compression ratio about a whole point without creating detonation risk.
Andy.