Synth / semi synth isnt really an issue - its the viscosity rating Thinner oil, especially when hot, flows well, but it doesnt have the 'body' of thicker oils, so it flows away again faster, hence doesnt have quite the tenacity of the thicker oils, so it could be argued, lubricates less - imagine the difference between water and cooking oil Thin oil also has a tendancy to "foam" under high revs / agitation, so tappets can get oil/air mix which doesnt work effectively - thats why high revving engines switch to solid lifters. So, too thin - it lubrictes well, but less as it heats, and tends to