I don't want to hijack the existing DT125B thread, so thought I'd start a new one.

Original petrol taps (petcocks) didn't have a filter on the taller "main tank" pipe and reserve was just a brass (?) filter. Nowadays, a lot of these filters have rotted away. I have bought sections of brass filter gauge off fleabay, cut these to length and soldered in place, to create a filter on both the main tank and reserve.

Two stroke oil tanks with a banjo bolt fitment originally had a filter in the bolt itself but, like the filter for reserve on the petrol cap, these can rot away. I am currently having a new filter fitted to my own banjo bolt. Later tanks, with a push fit for the oil line, had the filter inside the tank. I guess these could be replaced by removing the sight glass and gaining access that way, but I'm not sure - some folks have fitted inline oil filters.

It wasn't until a year or two back that someone mentioned oil tank filters. & i had to admit i never knew they had one. Me probably thinking oil--thicker than petrol--would struggle to gravity feed through it. Hate to say i've still not seen one. Should check my AT1 before its too late although pump's still feeding--I hope!!. Only been 50 years--Umm.
Back in about "75 on farm bike 100's [AG100's] that came with a big fuel tank & a "big oil tank" which i guess took for ever to need topping up--we had trouble with some that were using outboard oil in the oil tank & it went a bit like jelly. was horrible dark blue oil i guess for premixing. Not sure why i'm thinking this but seem to recall it smelling a bit like ammonia. Maybe i'm thinking of Janola bleach stuff smell.

I guess that they might only rot if they've been used, but then stored without any oil/petrol in for years. If your AT tank has had oil in it for most of its life, then probably no issue.

My repaired bolt.

Photos now working, thanks Mako! 

  

I don't have a filter on my AT1 nor my RZ350. Both haven't had an issue. I do run a external fuel filter.