I'm the one that's been saying M1 xW30 is almost 20wt, and it's true. HOWEVER, according to the ACEA A5 spec, it will not thin to 20wt, meaning it won't thin hardly any. That's good.
M1 TriSyn 0W40 was on the light end of 40wt, but their SS 0W40 is right in the same range as the rest of the 5W40 and 10W40: 14.4 cST@100C
You asked about the what current spec would be most like the old 10W30. I have no data but I suspect that before any "Energy Conserving" tests were considered, most 10W30 were probably around 12cST@100C out of the bottle. Right where Pennzoil High Mileage, Quaker State High Mileage, and Shell Rotella T 10W30 are now. Heck, even the Castrol GTX ISAC GF2 10W30 was 11.3 cST@100C and it did include an economy test. In a synthetic, AMSOIL SAE 5W30 is right there at 11.7 cST@100C. If I could get AMSOIL otc that's what I'd use.
If it's 10W30, it's API SL, and it's NOT ILSAC GF3, and no API STARBURST symbol, then it may be what you're looking for. Check the specs to be sure.