We have some 15 story buildings in Ontario with 6", I have done 4 stories myself. As long as you engineer is ok with it, you should have no problem.

Paul Stevens

I agree with Paul consult with your engineer. We have done sevaral homes that where 3 and 4 stories .One of the most determining factorsin using the 6" or 8",
is usally the backfill on the basement walls.

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STACKING THE CAROLINAS ONE BLOCK AT ATIME
TODD BIBBERSTAFF/ TB MACS ICF CONTRACTOR

As mentioned, backfill height and type of soils come into play. Another factor is continous length of wall. Long walls and deep backfills can easily be handled with a six inch core. On some occasions, a pillaster or buttress incorporated to the exterior, below grade portion of the wall will add strength and rigidity to long walls without having to go to an 8" core. As for height, over 15 years ago we built a building 56' tall on an old 5 3/4" Greenblock core with no problem.

stacking one block at a time...

i personally just build my house ( first timer ) with Quad Lock all in 8" from grade to 40ft high last blocks
( some parts are 12" because required to hold up weight ..but 95% is 8"concrete )

i personnaly choosed 8" because i would've save only 2500$ for 6" VS 8" and the 6" is way more solid
i don't recommend 6" for basement ..but what do i know

i'd say sum up the diff in price
and then u'll see if it is worth or not

8" is 35% thicker than 6" and is up to 70%stronger is some type of load calculations