(I copied this over from my 5w30 review because I talk about both products here)A lot of reviews here talk about the product itself, and I think they have hammered the point that RP is one of the best synthetics that you can buy, up there with oils like Redline. I purchased my '10 RX8 in May 2010 with 16 miles on the clock and have been babying her ever since. After a few goes at conventional oil to break in the engine, I switched over to the RP after some great suggestions by the rx8 community. In doing so I took care to be very meticulous about my oil changes, performing them every 3000 miles, and saving samples to send to Blackstone labs just to make sure that the oil was doing a good job and that my engine was healthy. I ran the RP 5w30 in the winter, and then ran their 10w40 in the summer and had absolutely no problems. Once I changed out my first interval of the 10w40, I sent the samples over to BSL (~$40 total to sample both) to check the results. Although the numbers were above average, the Viscosity numbers were a tad low .... not alarmingly low, but I expected them to be stable considering that this is a premium oil. The guys over at BSL assured me that this was completely normal, and because my car only has 10k miles on it now, it could just be residuals from breakin/conventional oil use/some 5w30 that stayed in the sump when i put the 10w40. In any case, I sent my reports over to the folks at RP just to have them take a look-see. Within a few hours I got back a reply from the RP team pretty much confirming what the guys over at BSL said. Along with that, they stated that they wanted to continue monitoring my use just to be 100% sure that their assumptions are correct. They sent me free sample kits and told me to send them some samples when I do an oil change again.There aren't that many companies out there that A. produce a fantastic product, B. advertise without bogus claims, and most importantly C. stand by their product 100%. If anything, this company got my respect and loyalty due to their immediate response and their handling of my concerns. Now again, let me stress, my oil analysis numbers were fantastic and were exactly what was expected of a newer car...but just because I had concerns the folks at RP asked me to send them over more samples so they can run an analysis free of charge.I hope that helps most of you that are still on the fence about trying this product even though it has a fantastic quality reputation. Hopefully this will convince you that the team also has great relations with their customers.