Well come on Paul, don't keep us in suspension, did you make it?
You can't possibly be on your original fuel tank , unless cleaned out & seriously treated, they must all be gone by now. From memory we all had them fail, rusting out from the inside, at between 18 & 20 years old. I guess those who replaced theirs with a new genuine tank, without extra treatment, could expect another round of rusty tanks about now.
I got an extra 10 years from my original, by repairing & treating it, but had to fit a new tank about 8 years ago, when the pick up & gage fitting started weeping at the tank. A sudden increase in fuel consumption from 8 to 14L/100Km gave me an idea I had a problem. Smart aren't I.
I found that Morey's upper cylinder lubricant is preferentially dissolved in water. It turns any water in the tank into an oily sludge. Although it would not be much help for the valve gear in this form, I expect it would no longer cause rust in the tank either.
I should know if it has stopped tank internal rust. I'll let you know in about another 10 years or so.