woah, you need tobe real carefull here.
How many mileso n the bike, anyway it s 20 tyears old so you need to be very carefull. If its been run on low detergent oil and then you go and fill it with modern high detergent oil you will wash all the shite thru the mains. Ok Dukes are more resiliant to this than ost bikes with their roller mains and big ends but its still not a good idea. You need to find out the history of what its been run on before.
Secondly. The beval have very fragile crankshafts, if i remeber right the Diremah had a sronger version than the old SS's but modern multigrades are apprently not that good. I recomend you seak professional advice here from someone how knows what their doing like Mick Wlaker or Italsport in Bury. Yagain you need ot bare in mind the bikes prior history.
Good luck.
Kev@eddnet.com