AO Platinum Member
Join Date: Feb 2002
Location: The land that Time forgot
There was probably a change to the emissions around then (EURO3 to EURO4?) so AR probably stuck the thinner oil in there to lower the cold-start viscosity... The reduction in drag might have been enough to sneak through the exam. The change to synthetic was probably just because 5W40 semi-synthetics are pretty rare.
Long change intervals are "okay". Tarquini, my old 155 had 12k intervals using 10W40 semi-synthetic and that's all.
When the head gasket failed at 198k I had the heads off. Wear was really minimal.. but there was a lot of gum and varnish in there and the oilways were partially obstructed.. though I should stress the gum film was maybe 0.5mm thick in an oilway typically of 5-7mm so less than 10% obstruction.
Given the mileage, I'd say it's probably impossible to gum up an engine enough to wreck it, if you carry out 12k changes.
These days I change the oil more often and run semi-synthetic "for diesels" since they have a higher detergent content. Synthetics also have the same high-detergent content (look for ACEA4 if you want the most.. though ACEA3 is more typical) and obviously don't break down like semi-synthetics, so they probably do last 18k and leave the engine clean and tidy.
No bullets for Chaingun..