Suzuki uses the Japanese Industrial (JIS) metric standard, which is a bit different from metric as used in the US or Europe. Regrettably, this makes metric a bit less of a standard. JIS bolts have finer threads (above 8mm) and reduced head (wrench) sizes. There's a useful table at: Bolt Depot - Common Metric Thread Pitches
Brent at TrailTough once pointed out (Thanks! Brent) that it's desirable to keep with the JIS sizes to minimize the number of wrenches you'll need to carry on the trail. I've also found JIS bolts at NutsandBolts.com
Metric class 10.9 is equivalent to US grade 8
Metric class 8.8 is equivalent to US grade 5.
There's also a stronger metric class, 12.9, mostly used for socket-head cap screws. For front hub bolts, I'm using the McMaster-Carr zinc-coated class 12.9 socket-head 8 x 25mm cap screws #95263A633 found at: McMaster-Carr
'cause they are even stronger than metric class 10.9.
The nuts on my (large) flange driveshaft bolts have a special captive washer and I've not been able to find them new, except possibly at Suzuki. Suzuki uses a lot of flange bolts and nuts, which increases the bearing surface area under the head/hex, while keeping to smaller, lighter hex (wrench) sizes. I kinda like that.
It's OK to use a stronger metric class, and I'm moving towards class 10.9 bolts whenever I have to change something. There's a good article on this subject at: ROCKCRAWLER.com - Grade 5 vs. Grade 8 Fasteners
In the front of the (downloadable) FSM there's a discussion of Suzuki bolt-head strength markings, which differ from those in the US.
Ordinary stainless is not quite as strong as class 8.8/grade 5. More and more I'm moving towards the exotic high-strength ARP stainless hardware, which is somewhat stronger than metric class 10.9. Most of the ARP product line is available from totallystainless.com. It is somewhat expen$ive, so I use it selectively where corrosion could be a concern.