ive heard that you drain the old oil, fill the oil pan with diesel,
Not sure what you mean by "oil pan". You drain the oil sump by removing the plug at the bottom.
As yeah right said, let someone else do the diesel thing. Too scary for me.
You can use a Woolies oil and a good engine flush additive and drain that, or just put the engine flush in now and then drain it. You must use a new oil filter before using the flush to catch a lot of the crap, and then put another new one on when you put the good oil in. Don't forget to put the plug back in before refilling with oil.
On the oil to use, I agree with the others, a heavy Penrite oil will suit that motor.
And if its an auto, don't forget to check your auto fluid level.
Hope she goes good