You oil crankcase has a capacity of about 7 quarts. Your oil light will probably kick in at a level of about 5-1/4 to 6 qts. I doubt that you engine would rumble that bad at those levels. Sounds like something else is going on. When is the last times you had your plugs checked?

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'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)