David, if I understand you correctly, you mix oil grades?
I have always read not to mix oil grades.
I figured it was meant to maintain
homogenous Temp. protection, but who knows?
As for the oil usage, no educated guess from me.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD