You are comparing a 10 and 15 w40 oil right? They are thin and thick 40wts. The difference is at 150 deg C. You are probably correct that the vii's effect is more noticible as temps increase, but at some point they should breakdown thermally. As you know vii's don't lube so if they are stiil good but the oil is turning to gas, can they still thicken it up? Perhaps best to have real all be it thin oil the 40wt. mono in high temps, when vapor finds a condensibally cold surface will turn back to oil, the thin base will do the same but may not carry along the needed vii's in the vapor, and not be thick enough as a result.