The bike should have an "oil filter" screen at the bottom of the injector reservoir. Maybe your friend was referring to that?
As far as a pre-mix ration goes, 32:1 will suffice.
Damage from the wrong ratio could be as simple as fouled plugs and too much smoke. Running too rich will also quickly carbonize your top end.
Running to leaning will quickly wear out rings, seize your top end and maybe the bottom end.
If you go the pre-mix route, I'd definitely recommend using a ratio-rite.
Excess smoke can also be a symptom of a bad clutch side crank seal.
What year is your DT? Is it a reed valve model?