*1993 Mercedes 400E, 127,xxx miles. 4.2L, V8 (M119). Owned for 6 years, plan to keep indefinitely. Daily driver, currently 12k/yr. No oil leaks. OCI 5-7,5k mi. Been using various synthetics over the last 28,000 miles, since I acquired her. I drive in a very brisk, but (mostly) mature manner. Hot and humid summers. *Original manual calls just for whatever API was current in 93 and 10w40, 15w40, 5w50, 20w50 for all temps. There's a 5w30 in there for up to 30C ambient, if it has a precursor to A3/B3 spec. So I'm aware that, technically, using today's lowly 10w40 dyno SN will exceed factory requirements. I'm also aware of the Walmart Mobil/Castrol 0W40 thesis. With the above out of the way, I'm curious: 1. Is 229.5 oil there most optimal choice for an engine developed in the 2nd half of the 80s? Any merit seeking out 229.1 or 229.3 oils? 2. Castrol has a 0w30 with a 229.5 certification, which means it has same HTHS as 0w40 and 5w40 229.5 oils. Would this not be the most optimal choice, given same protection as heavier grades, but with some, however minuscule gains in fuel economy and power? 0w30 is not in the original manual, neither is 5w40 and 0w40, but it's been 27 years since... 3. Is there any merit in using synthetic high mileage 10w40 oils that don't have any manufactures certifications instead of 229.1/3/5 oils. Again, there aren't any oil leaks. Thanks!