Are you talking about a screen that is under the oil drain cap on the left lower side? That screen is just meant to be cleaned, and put back in. The real oil filter is under the small right side cover, you loosen the 3 allen bolts, as said earlier. Good luck, and if you run into any more questions, dont be afraid to ask.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes