UPDATE.

After working on the car, early this morning. And checking oil (I left it down a qt before to check for similar symptoms of valve seals), I concluded that the cause of the shaking, rocking and rolling, was the EGR. After another 130 or so miles total, the oil is hasn't dropped an ounce. Looks like my 350 engine might be in the clear after all, with new valve seals and a new EGR (or I'll just rip it off).

Thanks for all your suggestions, but I think I'm all set, now.

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1987 300SDL (324000)

1986 Porsche 951 (944 Turbo) (166000)

1978 Porsche 924 (99000)

1996 Nissan Pathfinder R50 (201000)