Here in Aus there is no mention of 8volt battery in any battery listing. If you need 8v you have a problem elsewhere in the system, fix it and stay with 6v.

Tony

@olsarge I put an 8 volt battery in my 49 GMC that I had a 54 engine put in it. The old mechanic told me that it would start the 15 volt engine but I will need to adjust the voltage regulator.  I found that if I had it adjusted to charge too much then I would burn out head lights and if I did not have enough then my battery would be too low to start the engine. It was kinda hard to find the sweet spot

FWIW, I have run 6volt tractors on 12volt batteries for over 30 years. However, I put a 12 alternator on them, and there was very little else on the tractor except ignition and light, which were converted to 12v. In all those years I never experienced an electrical issue with the starter, but I did twist the armature winding loose from the shaft on one starter, due to the higher torque involved. I also discovered that tractors with magnetos would not start reliably because the starter spun too fast for the impulse coupling to function correctly. I wasn't looking for an advantage function wise, as stated ElseWare, a 6v system works just fine if maintained well, I just wanted to get away from two different batteries for all the equipment. 

I would stick with the 6v in the cars/trucks we are dealing with here. my two cents