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Originally Posted by TrackClub
No 5w30 is certified as dexos2.
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Mobil 1 ESP 5w30 is Dexos 2.
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I would even argue that 5w weight is a better choice for summer only driven cars, as it will provide for better protection at start up.
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I don't follow you here. The issue with protection at startup is not about how thick the oil is, but how easily it flows and starts protecting after the engine is started. Since all oils are much thicker at cold start* than when at operating temps, thicker viscosity is the enemy! This isn't just in winter temps: it matters for a "cold" start on a warm day, too. For example, the kinematic viscosity of M1 ESP 0w40 is 69 mm2/s at 40C (104F), but only 12.9 at 100F. OTOH, M1's "Formula M" 5w40 has a viscosity of 79 mm2/s at 40C, meaning it will take 14% longer to reach critical parts on a cold start, even on the hottest summer day. It has almost the same viscosity when hot, though, at 13.2. Will that 14% extra viscosity mean your engine will blow up if you use 5w40? No. But it does mean you add more wear on every cold start compared to if you used 0w40, even on hot days.
The M1 15w50 has a 40C viscosity of 125! So it will take almost twice as long to reach all the moving parts on a cold start in the summer compared to 0w40. That's why GM says you can't use it for street driving. But is' 100C viscosity is 18 and its HTHS is 4.5 (vs 0w40 ESP's HTHS of 3.53), so it does provide better protection on track.
*"Cold start" meaning when the engine hasn't been warmed up yet, not meaning a cold ambient temperature.