Personally I would NOT use a 15W50, especially not in winter in PA.

The 5W40 is what I have used in my '98 and what I would recommend you use in your '99. Either of the 0W40 or 5W40 is CF rated which is diesel approved but the 5W40 is CI-4 rated too, which is a higher level of diesel rating.

The confusion might be because the CI-4 is rated for ultra low sulfur fuel which is what we are seeing now...so for that reason I would stick with the CI-4 rated 5W40 from here on out.

FYI - While I have not seen the new M1 5W40 on the shelf at Wal-Mart yet I was able to order it by the case from my local NAPA store last month and they got it in one day from a local oil distributor...it is the "Turbo Diesel Truck" Oil which used to be the "Truck/SUV" oil. From everything I have seen and read it is the exact same oil, just labeled differently now but if you need it just order some. A case is only 6 quart bottles.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz