Socket head cap screws (SHCS) are grade eight.....there is no such thing as grade 6. The confusion may be due to hex head grade 8 bolts (SAE) have six radial lines on them to identify them as grade 8. Three lines designates grade 5, this is sae and nf, nc, USA marks. Metric goes by numbers on the head.

Here is a simple chart: http://www.boltdepot.com/fastener-information/Bolt-Grade-Chart-A.aspx

Michael.............still serving my 30 year machinist apprenticeship