Hi, I switched from Kendall's 15W-40 mineral to Teboil's 5W-40 Full Synthetic S-HPD oil, which is CI rated for heavy duty trucks. Mainly because of cold winters that we have. Before the change i used Auto-RX to clean up my engine and for the first batch of synthetic oil i took it out after 2000 miles. After every oil change i opened the oil filter to see how much dirt there is, but actually i didn't find any visible particles from it. And after that break-in for synthetic oil i have been driving about 5000 miles change intervals, could probably drive a longer intervals, but i haven't got any chance for a lab tests. Last summer was a record hot summer here and first i thought that i need to change to little thicker oil, but then thought that i'll wait and see how the thin oil behaves and as a result i didn't found any reason why i should change to thicker oil, because engine was running smoothly, quiet, no leaks and didn't consume any oil even thought it was 38 Celsius (about 100F). Now saturday here was about -36 C (33F) and truck started at first start with one cycle, but then again i have Eberspächer (5kW) doing preheater job and i kept it going for 50 minutes before start. If you find one for the right price i really recommend installing one of those. As for oil, i would have tried Amsoil synthetic, but they cost about 28€ a liter in here so i think i don't have to say anything else, but Teboil Synthetic costs about 6 € a liter...