The reason M-B tells you to use only synthetic oil in the car is because they ran into trouble with the FSS when they said it would work with either syn or conventional oils and then engines started getting sludged up because the FSS was not correcting properly and was letting the engines go too far between changes on conventional oils. For that reason M-B extended the warranties on the early FSS equipped cars and mandated that they all use fully synthetic oils.

I don't think you would be at great risk going to 0W40 now and that's what I would do. As a precaution you might think about doing a second change after 500 or so miles, or at least change the filter again, just in case, since you don't know the history of the car and there might just be some sludge loosened up by the syn oil...but that's really just being extra cautious.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz