73 Corvette with unknown oil mileage report

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Nov 30, 2010
Northern KY
Hello everyone, I need some interpretation on the oil report below. This is a 73 Corvette that my uncle is considering buying. No one knows how many miles are actually on the oil, so I guessed at 1000. It could be as little as 500, it could be as many as 10,000. I have no way to know. I also do not know the brand or viscosity. I know the oil was black when I pulled it. The car was warm, but had sat for about 30min, and that I sampled the first oil that came out of the drain. I basically just loosened the plug and let it dribble out. I am curious if these numbers seem worse if there is only 1,000 miles on the oil than if there were 10,000 miles on the oil? Thanks everyone.
I'll take one https://images.search.yahoo.com/search/images;_ylt=A2KLfSK9jI1aWhEAONxXNyoA;_ylu=X3oDMTByMjB0aG5zBGNvbG8DYmYxBHBvcwMxBHZ0aWQDBHNlYwNzYw--?p=73+corvette&fr=dss_yset_chr
Too bad they didn't do a TBN on the oil. That would have been a good marker of very old/tired oil vs. something only 1K-5K miles old. Judging from the low calcium, moderate magnesium, phos/zinc, moly/boron mix, it resembles a 5w-30 Mobil 1 oil. If so it started out life around 11.1 viscosity. So that could mean a fairly used up oil to drop from 11 to 9.67. Certainly not 1K miles. Maybe 4K-8K? I don't think this is 1K mile oil as everything seems to point towards it being a Mobil 1 - 30 grade. PQIA Isn't this a flat tappet engine? And if that's the case, you'd want more zinc in there than M1 5w-30 could provide. Some Castrol oils also have higher magnesium though this doesn't look like one of them with that low a Calcium number and lower viscosity.
OK, uncle buys Corvette. How many actual miles are on the chassis and engine? Auto or stick? What does the tranny fluid and power steering fluid look like? A lot of corrosion around the battery terminals? Is the rear end full? Look critically at the whole, not just the oil. Pull a valve cover. That will tell you more about the engine than a unknown mileage oil sample.
Presence of Mo, Boron , Ca and Mg indicates it's a Mobil 1. High metal wear rates of Fe, Cu, Pb etc indicates excessive long OCI of 15000-25000 miles of several years leading to extensive oil oxidation and viscosity thickening to 9.67 cSt [email protected]*C. Thus, oil grade was likely a xW20 of virgin [email protected]*C of 8.5-8.8 cSt. Suggest 2 short OCI's of 5000 miles each with a mineral/blend xW30 for cleaning, follow by any full synthetic xW20 of choice. This engine should be good to go.
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Thanks everyone. To answer some questions and provide more info.. Car and engine have 35,000 miles. It is a 4spd manual. Car is in very good condition, but it sat for most of 20 years before being pulled from a garage. I am thinking some of this wear might be from dry metal parts. Heres some pics of our inspection at my house on Sunday.
That car is pretty darn mint. Someone obviously cared well for it before it was put up. Not likely they'd park it with an OCI of 15K-25K miles. The recommended oil for that car was never a 20 grade. Why would anyone use that? And a 4 speed no less which is even more reason to use a 30 grade or higher. And OCI's for that car were originally around the 3K mark. Why would any caring owner push a 25K OCI on such a car, even with modern synthetics? It's a 4 barrel carb'd car which fouls up the oil a LOT sooner than today's FI/DI cars. More than likely it's a 30 grade oil in there and probably with average miles for OCI's of the 1990's to early 2000's. Only 2 engine choices for 1973 Vette....350 or 454. There could be 10,000 miles on that oil. But probably more likely it's only 2K-6K miles.
From what I understand, the car was bought by a young guy in the late 70s/early 80s who had it for a few years, but then died unexpectedly. The car then went to his father who kept it as a sort of memorial to his son and never drove it. When he died roughly 20 years later, it was sold as part of the estate.
Blackstone once wrote on one of my reports that old oil can turn “acidic” and cause these sort of readings. (search post started by me, its probably here, think it was the F100.) SBC is not expensive to overhaul even if it needs it, I’d buy the thing if he wants it, it looks like a nice car... I’ll probably offend the purist but it needs a 383 with a roller cam anyway... cool
Originally Posted By: 69GTX
More than likely it's a 30 grade oil in there and probably with average miles for OCI's of the 1990's to early 2000's. Only 2 engine choices for 1973 Vette....350 or 454. There could be 10,000 miles on that oil. But probably more likely it's only 2K-6K miles.
I stand corrected. You're probably right with a xW30 on 6-10k miles (with shear thinning being reversed,and onto thickening) AND many long years .. Edit:None of fuel dilution btw.
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Blackstone hangs on to samples for a while. If you want a TBN, give them a ring or email and they can run it for you. Extra fee of course but it might be worth it here. The you could run a new oil for a prescribed time and send that in to see if anything has changed. Beautiful car.
Originally Posted By: dishdude
Sweet car. Buy, change fluids, enjoy.
Yup. And thoroughly flush/clean the fuel system from fuel tank to carb. With that nice original paint and ultra low miles, that baby deserves to remain essentially factory stock. So few like that still survive...most being modded or altered along the way.
It's a flat tappet engine, I would look at Lucas Hot Rod & Classic Hi-Performance Motor Oil SAE 10W-40. tbn/9.3 b/22 ca/2307 mg/2 mo/846 p/2373 z/2045 ROD
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