We're definitely NOT.
But I SINCERELY hope you don't have issues with your choice, Larry.
OK Barry maybe were not both thinking alike... what is the answer to
this Oil University question???
While racing at 95°F, mid-summer in Florida, which of these
synthetic oils gives the best protection at 302°F oil temperature?
A. 0W-40
B. 5W-40
C. 10W-40
D. Straight 40 grade
E. They are all exactly the same
Answer:???
Incomplete question, Larry. You don't mention which oils they are, what type of racing, etc.
If I were racing a 24-hour race where endurance of the oil is crucial in high temps of 95 F or higher, I'd use a straight 40W synthetic or the 10W-40 because they will tend to shear less over time (all else being equal) than the wider range oils.
This is assuming that the nominal 40 grade oil is appropriate for the motor in the first place in these temps.
The cold oil performance is irrelevant here, as the oil would only be cold before the start of the race, and so it would come down which oil can survive the punishment the longest, without degradation, and as you know, Larry, the narrower range spec oils require less VI improvers than the wider range oils, which can degrade faster.
If the 40W oil was a dino oil, and the 0W-40 or 5W-40 were a good synthetic, then I'd likely choose the synthetic, but again, with your incomplete question, I have to make the assumption you're talking about similar types of oils, with similar general package makeups.
If the racing were short-distance drag racing type stuff, then I'd likely choose the 0W-40 due to its lighter viscosity when not fully warm (many drag motors run with cool oil not yet fully warmed up), and to [censored] with the long-term endurance of the oil.
If I were riding on the street in Florida, then I'd choose a good synthetic of the recommended grade from the manufacturer for that average temperature, and that would avoid the use of the straight 40 grade oil, due to the number of cold startups one will encounter.
So, not sure how this helps you, other than I'm going to guess that you are going to suggest that all of the oils offer similar performance due to being nominally rated the same at their warm spec.....to which I'd reply that you need to be careful that actual oil viscosities are hardly ever the same as their nominal viscosity ratings, and there are a whole BUNCH of other performance and usage criteria to be concerned with, when comparing oils, than simply their nominal visc rating.
More specific to your particular situation, if you asked me which oil I would run in your RC45 in Florida or California in mid summer, between the Castrol 0W-30 Euro oil (GC) and your chosen Mobil 1 Advanced Fuel Economy 0W-30 oil, I'd absolutely ALWAYS choose the GC oil, despite them having the same nominal rating.
The GC is a very tough, EUro oriented oil with HTHS > 3.5 (although Castrol never fixed their PDS documentation) while the M1 is only 3.0. That is too thin, IMO, for an application where Honda recommends a 40 or 50 grade oil and does not recommend ANY 30 grade. The GC will almost certainly be fine. The M1 Fuel economy oil....is highly questionable to me.....and not worth any potential issue it may cause in your application.
