If you started with the oil level at the mid-point of the dip stick, you have 'lost' at least a quart of oil in 400 miles! That is excessive oil loss. My 300E uses about a quart in 5000 miles.

I'd guess you have a serious leak and maybe some oil coming down around the intake valve seals and then being burned. I'd expect to see some blue smoke out the back if the oil was burning. Have you looked at the condition of the spark plugs? If they are oil-fouled you know where the oil is going.

I wouldn't run the engine when it is that low on oil. keep the oil level up to about the mid range of the dipstick and find out where the oil is going ASAP.

I once had a 1953 Chevrolet which burned oil badly. On a trip from California to Iowa, I carried a case of oil in the trunk and added a quart at nearly every gas stop!

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog