Mileage too high for Synthetic?

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Jan 12, 2003
Hello Everyone. My 78 Trans Am has 67k on its engine. I was just wondering if that is to high to start running on synthetic. Would it cause leaks? The engine is a 400ci Big Block, All Stock. Except for a K&N Air Filter. Any input would be great. Thanks
From what I have read here it's not too late assuming the engine is in good shape and not leaking oil. An Auto Rx treatment would be in order prior to switching over. If you're not familiar with it do a search on the site for Auto Rx.
Should be no problem. Although some folks have had some seepage on gaskets. I have switched numeroud high milage cars and have not had a problem.
I switched at about 150K in my Pathfinder with no issues. That was close to 50K miles ago now.
Both my daily drivers ( 98 F-150 and 97 Saturn )were switched to Mobil 1 10w-30 at about the the 90k mark. Neither leaked oil before or after the switch. They both ran on Mobil 1 till about 120k when I switched them to Schaeffer's #700 15w-40. Now the Saturn was a typical oil drinker at the switch. That slowed a little with the Mobil 1 and then was cut in half with the Schaeffer's 15w-40.
Something to bring up with people MUCH more experienced than i, isn't it possible this car could have a rope main seal? From what i knew, the esters in modern synthetic oils swell rubber seals, but not rope. Anyone care to pick up where my very limited old-car knowledge leaves off? ferb!
Synthetic Myths!! As long as your seals/gaskets are in health. You can switch to Synthetic Oils ANYTIME! In an issue of hemmings last year they had a 1957 Corvette with an engine that was rebuilt once 2O years ago. They switched to Mobil 1 and tested the consumption and leakage. Needless to say consumption was negligible (1qt/3000miles) and the leakage was not to different from dino oils. Case #2 My parents have a 1993 Oldsmobile Ninety-Eight Regency Elite full size Sedan with the 3800 Series I V6. It was run with sometimes regular and sometimes neglected oil change intervals on dinos from factory all they way to the Summer of 2000. At that mileage of 151,000 miles the engine never had a problem and was still very smooth. After doing a lot of reading I switched to an OTC Mobil 1 TriSynthetic 10W30. I thought running the Royal Purple I use in my Camaro would be overkill *lol* Anyway here is where the interesting stuff starts: I did add 2Qts of Gunk Motor Flush to engine before the Mobil 1. After the oil change and within 4days of driving the Mobil 1 looked a NASTY jet black and was gritty. The oil smelled like tar! We changed the Mobil at 2000miles. The same thing as described by the 1st oil change happened. By the third Mobil 1 change the oil essentially looked normal. We usually follow the engine oil life monitor and depending on conditions that means a change every 4-5k miles. Leakage actually decreased since spotting on our driveway is not as much as before. The oil looking like tar and assumption is all the years of skank and crud from the dino oils got solvated and cleaned off by the Mobil 1 (or much of it at least!). When in doubt I woudl invest in Auto-RX (via two treatments with dino) as a means to pre-clean your engine before your introduce the aggressive synthetic oil chemistry. That was nearly three years ago and now the "Old-Faithfull" 3800 Engine has 181,000 trouble free miles. Funny, all the mechanics "in-the-know" told me it would be a grave mistake to change at such a late point!!! LOL Sunil-
I switched to Mobil 1 in my '93 Grand Marquis with 89,000 miles. That was four years ago and the car had 201,000 miles when I donated it this month. Still ran fine.
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