Ray Knight: I have no experience with Optima, so my comments are regarding 12V jumping of 6V cars. I will try to separate what is written from my own experience. There are several problems and/or potential problems with jumping a 6V car with 12V.
With these cautions you have to make your own decision. I will not jump a DEAD 6V with a 12V, I will try charging first. I do however rather routinely jump weak 6V with 12V, have for 35 years. I do wear glasses or goggles, I do make the last connection to ground remote from the battery, and I always look away from the battery, and have good ventilation. I don't leave it connected very long (remove as soon as starts), and I do turn radio and all lights off.
I always thought it would be nice to have a set of cables with inline sealed switch, but not aware of any being made. If you accept the risk and precautions to jump with 12V you then have the option of carrying one of the new portable boost battery units, which I now do.
Geoff Fleming: It is done all of the time and has been for decades. Just use normal precautions and once the car starts, disconnect the jumpers. Be sure that all power using appliances are off during the process so no excess voltage is transmitted to them. It is best to apply both cables to the 6 volt battery first, or one to ground and the other to the positive termnal then make the attachment to the 12v source.
Steve Proctor: I have always gotten a bit of a spark when connecting or disonnecting the jumper cables. Therefore, since batteries can generate hydrogen gas, you want to make sure the last connection you make is not at either negative battery terminal. Connect positive to positive, then negative to the 6V battery and finally, negative to a frame member of the 12V car. That will keep the spark away from either battery and is much safer. I have exploded a battery before in a different scenario and it is a scary experience.
Second, the battery cables may get warm during this process, so once connected, I move quickly to start the car and to disconnect the cables as soon as possible after the car with the sick 6V battery is started.
Harry Pellow (July 23, 1999): Sometimes it IS necessary to Jump Start a 6-volt 356 with a 12 Volt Battery. And Sometimes you must do it YOURSELF, like when you're alone, so that you don't have the Luxury of having two people. I got my early Teenage experience in the Old Man's Junk Yard. Ever try to start a 6-Volt Ford with a 12 Volt Battery when the Ford is POSITIVE Ground? You learn fast in the junk yard.
My one-person, 12 Volt Jump to a 6 volt Battery procedure:
Voila! So far, about 100 12 Volt Battery Starts of a 6-volt car. No 6 Volt (or 12 volt) battery explosions yet!
Praise the Voltage gods. And: Keep the 356 Faith Maestro