Many threads regarding proper way to check oil. Marshall always endorsed overnight cold check. He submitted up to a quart of synthetic oil would adhere
to the internals. My 603 is driven every 2 weeks. oil level generally right at 1/2 way. Recently 3 weeks passed between operation. The oil level was just below full. 2 weeks back to half way. R.I.P. Marshall Booth

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1990 350 SDL
Currently: 180,000 miles
Factory rebuilt bottom end rods, bearings and head gasket @ 75,000 in 1997