STICK WITH IT...there is no reason to change as long as you change every 3K miles

 

Go ahead and run the 25w-50. Change the oil every 3k miles, don't exceed 5k. The oil additives may breakdown a little quicker then oil formulated for automobiles, but you will be fine if you change it every 3k miles. It is good oil, and will serve it's purposes just fine for your Disco.

 

The only reason you would need the rust inhibitors (oil prevents rust anyway) is because a boat engine may sit unused for months at a time.
Its free, go for it.

 

The fact that its free was the only reason for even thinking about it. Once it warms up I'll try it out.