May I bit to thin with the miles ,try a slick 50 treatment or as you said a good 10w 40 , don't have a problem with mine on 5w30 but miles quite low 50k,
Just got the rest of my engine plastics hydro dipped in gold carbon @aqua grapics in blackwood Gwent done a spot on job, now just to change all the bolts to stainless steel, proper clean ready for first show.