To the OP: No worries short term. Your engine hold 10 quarts and adding 20 percent of another viscosity an brand isn't a huge issue. Slightly incorrect oil is way better than low oil. Oils are all pretty compatible these days so unlike many years ago, there is little danger of additive clash. It might shorten the working life of the oil a little because the amount and types of additives in the S-rated (gasser) oil is less suited for diesel use, but if you do a "normal" 4K OCI (Oil Change Interval), I don't think you should have any trouble... especially if you top off again later with the right oil. Yes, you should be admonished for not having a jug of oil behind your seat but we'll let you live ( : < ). I've owned my truck going on 30 years, purchased with 7K miles, and from day one, it always used oil in small to moderate amounts... no matter what type, viscosity or brand I put in. It's normal for engines from this era and these engines in particular. It gets worse as they age and you just have to keep up with it by frequent checks. In the old days, people were conditioned to check oil religiously but today's drink-no-oil engines have gotten people out of the habit if they happen to start running an older generation truck.
My Hijack, but its related:
what i read, our engines drink rotella like a drunkard slamming a bottle of JD.
What you "read" is incorrect and I challenge anyone to prove it's true. I know that statement will bring about a barrage of anecdotes, ******** and repetitive old-man's tales (which I will ignore), but little in the way of science (which I will not ignore if someone can prove me wrong on that basis). If you know ANYTHING about oil, the composition of it and the general trends in the way oil is formulated by the different manufacturers, you will know this is just more internet nonsense. When I wrenched at car dealerships (including Ford), and this was in the era when most engines used some oil normally, we got a lot of complaints about oil excessive consumption. There was a whole rigamarole to verifying the complaint and often, very often, it turned out to be the customer; how and when they checked the oil, for example. With our trucks, the biggest mistake people make in this regard is "topping off."
On our trucks, it's two quarts from "add" to "full" and anyone that pays attention to the normal oil consumption will realize that that first quart will burn off much faster than the second. If you constantly top off from that midway point, you will think the truck is using a lot more oil. If you only add one quart when the oil level reaches add, perceived oil consumption will be less. This what I've done on my truck for the past 15 years or so. This was the common procedure back in the day, at the Ford dealer too, which is where I learned of it. There is no benefit or advantage to keeping the oil level at full all the time and a few disadvantages. Windage is one (costs power and speeds up oxidation by churning it up more) and oil consumption is the other, which creates combustion chamber deposits.
Another mistake is to check the oil level cold. There are reasons why they want you to check the oil on a warm engine (sitting on level ground... another common mistake) that has had minimum 5-10 minutes after shutdown to drain the oil from the upper end. Like anything, oil "expands" when it's warm. It's a small amount between 100 and 150F, say, but if you check it cold on a 30 degree day versus a warmed up engine, you will see a noticeable difference. If you observe, you can note what cold looks like versus warm and take that into account.
I have tried several brands of oil (Rotella, DELO, Mobil, Citgo, Royal Purple and, way back, Pennzoil) and none used noticeably more oil than another, with the exception of the 10W30 Pennzoil SF/CC which was the original oil recommended and really only a glorified car oil (it used more of that than the 15W40 SJ/CH I switched after a couple of years). This has largely been true with the two viscosities I have used as well, 10W30 vs 15W40... no significant difference in consumption and what I difference I did notice favored the 10W30 using less. I use 10W30 (Rotella T-5) exclusively now, by the way, but I still have a couple of cases of Royal Purple 15W40 synthetic so I will probably use that up before going back.
