Both of the synthetics you mentioned are fine oils. The M 1 turbo diesel lists the S designations so one will realize it is fine for gassers even though it mentions diesel in it's name. The T 6 has a more extensive listing of the C specs and also lists SM, which means that it also meets SL & SJ, etc. since the S specs are progressive, i.e. SM also meets all specs with letters that come ahead of M. You can't go wrong with either.

Both are year around oils and will work just as well in Texas in the summer as in cold climates in the winter. Indeed, the 15W40 standard oil you mentioned is a good year around oil in all but the very coldest areas. If you are in a climate where you get frequent sub zero F nights you could even find 0W40 oil that would work fine year around in that harsh climate. I don't believe there is any reason for using a different oil in the summer than in the winter, and I remember a time when we used 30wt in the summer and 20wt in the winter. Very different oils, and engines were worn out at 100,000 miles back then.

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'97 E 300 D