Back two months + ago I did a 5000K (3000 miles) service on my D21. I did the usual things like oil change, filters (fuel and oil), lube etc. I also sent the starter for rebuilding at that time and, while it was away, I drained the Trans oil and the Diff oil. I cleaned out both as best I could, washed them with brake clean, and let them both drain for two days. ( busy with other things and had to wait for the starter anyway) I installed Red Line 75-90 in the Diff and MT 90 in the Trans. (Can’t remember if it was Phillip or Al that suggested these, plus other brands to me). I put in Petro Canada Duron E 15/40 CI 4 engine oil. I also added some Stanadyne Fuel additive (Performance Formula) as per the instructions on the label.
Over the past 5000K’s I have noticed the following;
Easier start up with less smoke. No “Hunting” at low speed, light throttle, nearly to operating temps. ( Something to do with the CSD no doubt, but it has stopped). I have gained nearly half a gear climbing up the hill out of town where I live. I used to have to go up there in 3rd and could hold it at 80Kph but seemed to be too many RPM’s for my liking and she would smoke too!! Lots of black smoke. Now I can hit the bottom at approx 80kph in 4th and it will only drop to 60Kph at the beginning and crawl back to nearly 70Kph most of the way up with considerably less smoke. Before it was on it’s knees begging for 3rd. The shifts from 1st to 2nd do not “Touch” any more and the 2nd to 3rd is much better as well. In fact it shifts like a new transmission!.
Over this 5000K’s I have gone from 30 – 32 MPG’s ( imperial gallons here) to 35-37 MPG’s and, a couple of straight highway tanks at 100Kph, no wind, of 39 MPG’s. I do not have to “Pin” the throttle in a head wind to maintain 110 Kph and, the smaller hills etc around the area here I can pull most of them in 5th now and only drop 10 Kph. Far FAR better than it was before, and used to shift to 4th most of the time. The performance continues to improve as far as engine power goes.
I know that the Synth. Oil in the trans has made a HUGE difference just in how it shifts when cold. What to attribute the biggest change to I do not know, the Synth. oil or the fuel additive. I suspect that the internal advance mechanism was not doing what it was supposed to do because of the ULSD
I spoke with my dad and he said that when it was new (1987) he thought that it got around 40 MPG’s ( remember imperial gals. here) but really never paid that much attention to it. He did say that it did not do as well as his 84 with the SD22 in it, but it did make more power than it did, and, you can’t get nothing for nothing. He also lives at sea level and I am at 4000 Ft + so that will have a bearing on it as well.
I have installed Delo 400 15/40 for this oil change and I noted that the oil that came out on this change seemed to be a bit cleaner than usual but that may just be my imagination.
I continue to use the Stanadyne fuel additive. Paul
So how about Km's per liters (or Imp)? 
GM sticker on our new car has the estimated economy in L/100km AND in mi/gal. "Imperial" is N O T specified. The mi/gal HAS TO be in imperial as the numbers for that car in the US are different as far as MPG's go, The Canadian "Energuide" rules dictate it. I can scan the sticker and post if you wish. 
