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.