what weight are you running?
10w40 would be the go, Gulf western oils are pretty detergent heavy so running some of that at 5000km intervals should eventually free it up
otherwise you can do what i did to my 31
4 litres of cheapy GW 20w50 (~$13)
2 x Nulon engine oil flush
Dump current oil (save it if you want and put it back in after)
Insert cheap oil + both oil flush bottles
Run for 5 mins at idle, do not drive it!!!
lift the revs to 2-3000 rpm for 20-30 seconds
idle for 5 mins
lift the revs
etc... do it so its running for around 20 mins
dump the flushing oil, change the filter, put new/previous oil back in
party