Rock on Green!
My old bus was named Simple Green, I love green buses!
I would recommend Rotella 15W-40 for your diesel (and most of our other members' engines as well). Unless you've got an old 2-stroker, there's no better oil I know of for an HD Diesel...
As for the coolant, what color is the stuff you've got in there? I STRONGLY recommend you flush, or at least change, the coolant you have in there if you're unsure about when it was last done. Improper coolant maintenance kills more diesels than infrequent oil changes. Ask anyone. Healthy cooling systems are ESSENTIAL to a healthy engine.
If you're not flushing or changing the coolant, the kind you will need to use depends on the color (type) of coolant in there. If it's RED, only use RED (extended life) coolant, if it's green, only use Green coolant. I think yellow is a kind that can go both ways, i.e. you can mix it with green or red coolant.
And, just in case you're a noob to all this, make sure you only mix your coolant with distilled water, DONT USE TAP!!! it has minerals that could build up, plus, if it's fluoridated, it might enable the government to reprogram your brain and control your mind
Either way, if you are going to drain or flush your system, I would recommend going with extended life coolant, and make sure you change your coolant filter while you're in there!
Not tryna preach, but I've seem lots of great engines ruined by bad cooling system practices! Its a sad sight...
Best of luck on your veggie conversion, I had a great time doing mine with great results, I'd be happy to share any of my knowledge with ya! I love using old aluminum semi saddle tanks for veg (bolt em to your chassis under the body skirts--my last one held 150 gallons and only cost me $300 from a semi salvage yard ;)