I wondered this two when Jim Todd discovered the G10 he's selling is actually G11. Near as I could find out they are essentially the same, although G11 holds up better to high temperatures than G10.
Here's what this site,
http://www.plasticsintl.com/epoxy.htm, has to say about it.
NEMA grades G10 and FR4 Glass-Cloth Reinforced Epoxy - natural color is typically a yellowish to light green. The most versatile all-around laminate grades are continuous glass woven fabric impregnated with an epoxy resin binder. Epoxy resins are among the most versatile and widely used plastics in the electronics field, primarily because water absorption is virtually nil, rendering it an outstanding insulator. Beyond its electrical insulating properties, epoxy resins exhibit great dimensional stability (shrinkage is usually less than 1 percent) and superior adhesive properties. G10/FR4 has extremely high mechanical strength, good dielectric loss properties, and good electric strength properties, both wet and dry. The main difference between NEMA Grades G10 and FR4 is that FR4 is a fire retardant grade of G10. Therefore, FR4 can be safely substituted where G10 is called out, while G10 can never be substituted where FR4 is called for.
NEMA grades G11 and FR5 Glass-Cloth Reinforced Epoxy - natural color is typically yellow green to amber. This grade is similar to G10/FR4 with the addition of a higher operating temperature and some improved mechanical strength at elevated temperatures. The main difference between NEMA Grades G11 and FR5 is that FR5 is a fire retardant grade of G11. Therefore, FR5 can be safely substituted when G11 is called for while G11 can never be substituted where FR5 is called for.
This site,
http://k-mac-plastics.com/data sheets/phenolic-technical-data.htm, is a technical data sheet for various laminates. G10 and G11 are identical in all specs until it comes to flame resistance and temperature ranges. G11 pulls out ahead in that score. G10's operating temp rang is -100º to 285º, whereas, G11 is -100º to 300º. Their flame resistance as measured by bond strength in lbs is: G10, 2000 lbs; G11, 16,000 lbs. I'm not sure what all that means, except you got got one hell of a handle with either material!
