Yeah the bottles I have are bigger than 8 oz. add it anytime. I usually fill up and then put some in. Or take it with you and add it before you fill it.
Can it be added at any time or is it best when the tank is close to empty and ready for a fill up?
The "best" time to add it is when you're standing at the gas pump getting ready to refuel. Pour it in first and it gets mixed well when you add the fuel.
I use it every 5,000 miles, after each 5,000 mile service interval. Or use it once a year if you don't ride more than 5K/yr.
JUST DON'T USE IT IN A WEED-EATER or 2 cycle oil/fuel mixed-it will burn up the motor.
Long ago in our shop we had a few weed-eaters com in and they used SeaFoam-it removed the oil along side the cylinder walls and seized up the motor.
One came in, then a few more-after we asked what they were using for fuel, one guy said they added SFoam-after that, the same company stopped using it and we didn't have any repairs from said company.
Hmm, never thought of running this in a bike. I put a can of it in my 74 SCOUT II and it gave it all kinds of problem, smoked like the dickens and made it really hard to drive.