how can using 40wt shock oul mess up the threads?

I have simular shocks on my mtr-3...I found the caps would come loose so I emptied the oil, cleaned everything up, and then put some tire glue (super glue) on the threads then tightened them.. let them set fer a couple min...then filled them with oil... this solved the problem...

I believe Ive actually had 60wt oil in mine at one time..so I can assure you that putting 40 wt oil didnt mess up the threads (that is unless your shocks are made from less quality aluim then mine,, which I doubt)...