diesel oils need to have an API rating of CH-4 or better, the SAE rating can vary depending on application (though 15W40 is more or less a set standard here)
it depends on the engine, state of tune, service, environment, etc.
petron is not bad, but there are MANY alternatives out there which offer better protection for less money.
Delo gold isn't a benchmark at all (it's API CF or CG from what i recall). Here everyone uses it but it's not a very good oil anymore. Fuchs, liquimoly, Zic, GS, Mobil, Valvoline, CAT, Cummins & Petromin offer much better alternatives which are API CH-4 (sulphur content up to 0.5% weight) or better. As for mileage, read what i wrote above (to heart_break_kid).