Way back in the early 80's I had a relatively mild street 383 A-body (.474 cam, stock 906 heads, 4-speed, 3.23 rear). I would get about 13.8 mpg (mixed city/highway) driving with 10W-40 oil, but drop to 13.5 with 20W-50 and no other changes. I was surprised it made that much of a difference (and about 10 psi more oil pressure too).

OP, any more thoughts as to what the Blob was?