Mobil 1

[Charger1970]

I have Mobil 1 oil and a Hastings filter in my '05 HEMI Ram. It has 21,000 miles on the truck. How many miles can I go before my next oil change? 

[aifilaw]

without going into great detail here...
use mobil 1 synthetic, and change the oil every 3,000 miles as if you had regular oil.
My friends father was one of the higher-ups in sunstate (rental equipment of all kinds from massive tractors to lawnmowers) and they did a 2-year study and discovered that adhering to this precise plan increased their engine life an average of 75% beyond standard oil.

[dogbert22]

Charger1970,

I too run Mobil-1 in all the cars/trucks in my "fleet". While I agree that changing the oil, regardless of type, every 3K miles is probably the best way to go, one of the appealing reasons to go to synthetics is to try and extend that 3K limit outwards a bit. Mobil-1 sells and advertises a 15,000 miles-before-change oil now. I think what they found out was that the oil and its additive packages simply weren't wearing out with time, and thus the oil change interval could be extended all the way out to 15K miles. My personal feeling is that this is a bit much, as one of the"other"  jobs of the motor oil is to suspend/carry away normal combustion/products/moisture/engine bits which "hopefully" would be captured by the oil filter. I personally let the oil change roll out to 6K miles, but change the filter every 3K miles. While this may be overkill, it can't hurt. As a final aside, using semi-synthetic apparently gives you most (but not all) the benefits of a fully synthetic without the cost. Make your own by adding half-and-half Mobil-1 and Valvoline (good conventional oil).