[Lynn Eades]

Just to add a finer point to a part of Robs post.
Metric bolt tensile strengths are identified by "Class", where as American thread form bolts are by "Grade"
So a metric Class 8.8 is roughly equivalent to an imperial Grade 5 bolt. (using Robs example)
Rob, hybrid bolts go back to Canadian Carriers in WWII.Ford of Canada built carriers with BSF bolts with heads and nuts to fit American spanner sizes.
Now days the plating on a lot of fasteners tends to be for show. A lot of zinc washed bolts will be showing rust after sitting in a bit of water on top of a 44 gallon drum, for a day or so.
Some high grade fasteners like FX brand bolts (grade 10.5) have a gold color type finish on them that might be marginally better.
During WWII Cadmium plating was a finish commonly used. Dodge used the stuff on their bolts. 75 years later with the heads half rusted off them, those bolts , once cracked off, will undo with your fingers. Those bolts are the nicest I've come across in 40 years as a mechanic. Cadmium is a toxic heavy metal. It's not used anymore.
Be aware that torque figures are lower for plated bolts compared to black bolts.
Also, be advised not to use high grade bolts in a heat situation.