For an oil change the W123 82-85 300D (with a 617.952 turbo motor) takes exactly 6.5 quarts to fill it to halfway on the dipstick. Thats how I do it...no need to guess and check over and over.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car