It's the 40 that determines the oil thickness (or thinness) at operating temperature. Its the ?W that determines how much (or how little) that it thickens up in the cold. So a 0W40 is not thinner than a 15W40 at operating temperature.

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Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY