The 30/40wt designations are a bit too coarse to really call this out, but the viscosities of many certified 5-30/0-40 oils are a lot closer than you may get think. Taking M1 Euro 0-40 and M1 ESP 5-30, for example, the former is 12.9cSt @100C and the latter is 11.8. If you look at HTHS @150C, they're even closer at 3.6/3.5 respectively.