I used a mineral based 20/50 ( Esso ) when it was available , later a 10/40 ....... changed every 1500 miles as Laverda recommend , checked the sump strainer every other change ......... always as clean as a whistle ....... that was over 226,000 miles , no problems whatsoever ......
I wouldn`t fit an external filter .... firstly : I just couldn`t be bothered .... secondly : I wouldn`t want to place any additional restrictions within the oil flow , and thirdly : I don`t like the idea of extending recommended oil change intervals ......
What on earth was going on inside the head of the person who let that strainer descend into that state ? .....