Does it have much blowby? That could be where a lot of the oil is going. Every 5-6k when I do my 5w40 synthetic changes I usually need maybe 1-1.5qts of makup oil in that time period, and our '83 (with no blowby at all) needs maybe .5qt

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)