I run 15-40 with slick 50 in my 300E and it runs fine. The 20-50 is too thick unless you have major consumption with thinner oils.

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Thanks, Nels
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86 300E
93 W250 4X4 Cummins Power
72 F250 Highboy
89 S-10 408ci BBC
custom V Four Chopper
77 Peterbilt 352
83 Jeep CJ-5