the economics of it depend greatly on your situation..... to me its a cost to benifit senario.....
I drive worst case senario, 12,000 miles per year. at my average daily 44mpg thats 272.72 gallons per year, lets say worst case for this year, its $3 per gallon, so i spend roughly $820 per year on fuel. (rounded UP from $818 and change)
So running the waste oil would save me $820 a year..... thats not counting the investement in materials to collect, store, filter, and prep the WMO to run in my car (which i would never do).
As to the benifit....... thats the only one i can possibly think of.
a new injection pump alone is over $1000 dollars. a blown engine, probably between $2000 and $4000. If you cant do the repair work yourself and have to pay someone else to do it, its an hourly repair bill of at least $100 per hour, swapping an engine per the dealership book is most likely around a 10 hour job. swapping the injection pump is also probably a few hours worth of work.
Then lets look at the environmental impact of running the fuel, im not even going to get into the emissions, thats just a karma thing, lets keep focused on the economic sense of running the WMO.
Legal issues, while i dont know the exact amount, im sure there is quite a stiff fine for running unaproved fuels on highways here in the states. so youd have to look at those fines as well.
And from an economic standpoint..... I like that im supporting the manufacture of biodiesel, i dont even mind so much that the price recently jumped to $3.00 per gallon ($2.99). Im helping the local economy, im doing something good for the environment, and spending less than $70 per month to do it.
Youre also messing with someone elses income, the guys that recycle the oil, they gotta make a living too, by burning up their supply, they cant sell it and provide for themselves.
So even from an economic standpoint (let alone environmental) it just dosnt make sense. saving roughly $820 a year, even assuming all that gets fragged is the injection pump, youd have to get over a year of trouble free service, maybe 15,000 miles worth to justify burning the hexxus juice, which i just dont see happening. at the very least, its no where near worth the risk IMHO. And remember, thats just to BREAK EVEN. So all the benifit of saving money on fuel would go to repair the blown parts. You dont benifit, and everyone else suffers. Not exactly an attractive deal to me.
As to tntoskey - in your cummins, have you had it torn down and inspected to verify this "no problems" statement? Just cuz an engine keeps running dosnt mean it has no problems. not saying youre wrong, just curious.