Diehard
forstly see my question about the 748 you were offered.
You dont need fully synth in dukes, semi is enough. Check your manual for the correct type, but if you dont have it go here, and download both the workshop and owners manuals for your bike -
http://www.duc.nu/manuals/ you will need to check the filter number too, plus you'll get the correct amount of oil required from these manuals too (think it is around 3 - 4 litres).
When doing the change, make sure you remove the magnet attachment form the sump, which collects metallic particles, clean and replace it.
hope that helps