You have one chance to convince me NOT to put Castrol Syntec 10W-40 in my engine. I have a 1998 Subaru Impreza 2.5RS with almost 80k miles. The manual allows for a range of viscosities. I would like to use 10W-40 this summer. I would also prefer the oil to have ACEA A3 rating. Out of the oil available to me locally I can only find Castrol Syntec, Castrol Synthetic Blend and Valvoline Durablend to fit these requirements. I looked at Castrol Synthetic Blend and it seems to be a little too thick for my liking. Syntec, on the other hand, is on the thin side of 40 weight, which is what I prefer. The car has been run on Mobil 1 5W-30 since new. I almost decided to try Castrol Syntec 10W-40. I knw it's Group III. But it's cheaper than Mobil 1, so the price seems to reflect the cheaper base stock. Well, actually with the 5 quart canisters at my local WalMart Mobil 1 is slightly cheaper. But it is not available in 10W-40. 15W-50 is way too thick to my liking and 10W-30 does not have ACEA A3 rating. Maybe I should just use Mobil 1 10W-30? Or maybe I should use a quality non-synthetic, such as Motorcraft 10W-40. I use it in my other Impreza and it seems to like it a lot. What do you say? Syntec, Mobil 1 or just a quality non-synthetic? I don't mind changing it at 3k miles. For winter I plan to use Castrol SLX 0W-30.