If you had to choose only one

I have always used M1 0W-40 in my Mercedes. I've used M1 for many years. A plus is that M1 runs rebates often. Nice savings.

However, for the last few months it is hit and miss to find M1 0W-40 at local Walmart stores. So I've considered switching to Castrol 0W-40. It's a great product. I'm sure it would protect my engine just as well as M1. But Castrol almost never runs rebates.

So my advice would be to select whichever is available and fits what you like to spend.
 
I've used both in my Mercedes. Both have MB 229.5. In the old days, Mobil 1 lost LL-01 and Castrol still had it so some preferred Castrol. Some claim that's what Mercedes prefers in Europe. Now Castrol also lost LL-01 so they're both the same. So if the price of both were the same, I'd go with Castrol. But Mobil 1 runs rebates during the spring, probably in a few months and Castrol doesn't so if you can wait, I'd do Mobil 1 for the rebate. Mobil 1 makes oil for Mercedes in the US so Mercedes has the Mobil 1 recommended sticker under the hood.
 
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