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Amber Lamps · 05-07-2008, 09:01 AM

Popular Mechanics,
A: Early synthetics got a bad reputation for leaking. This was because, despite the claims of the oil manufacturers, the seal-swell characteristics of the new synthetics were different from those of the mineral oils they replaced. If the seal-swell rate was lower, the seals shrank and oil leaked from crankshaft seals and rocker cover seals. If the rate was higher, the seals swelled a little extra and the engine was tight. Then if the owner changed back to mineral oil, or added a quart when no synthetic was to be had, things got really bad. The crank seals had become worn, in their turgid state, and then relaxed. The valve cover seals were compressed when swelled, and when the different oil was added, everything leaked like, well, an old English sports car.

Fortunately, the situation has improved; you should have no problem switching back and forth. Adding a quart of mineral oil to a crankcase full of synthetic will be fine. Read the fine print — a lot of the "synthetics" on the market are blends containing a substantial proportion of mineral oil.

And that's the reason for not switching back and forth! Btw apparently this is from the 60s or 70s.....

05-07-2008, 09:01 AM	#39
Amber Lamps Moto GP Star Join Date: Mar 2008 Posts: 14,556	Popular Mechanics, A: Early synthetics got a bad reputation for leaking. This was because, despite the claims of the oil manufacturers, the seal-swell characteristics of the new synthetics were different from those of the mineral oils they replaced. If the seal-swell rate was lower, the seals shrank and oil leaked from crankshaft seals and rocker cover seals. If the rate was higher, the seals swelled a little extra and the engine was tight. Then if the owner changed back to mineral oil, or added a quart when no synthetic was to be had, things got really bad. The crank seals had become worn, in their turgid state, and then relaxed. The valve cover seals were compressed when swelled, and when the different oil was added, everything leaked like, well, an old English sports car. Fortunately, the situation has improved; you should have no problem switching back and forth. Adding a quart of mineral oil to a crankcase full of synthetic will be fine. Read the fine print — a lot of the "synthetics" on the market are blends containing a substantial proportion of mineral oil. And that's the reason for not switching back and forth! Btw apparently this is from the 60s or 70s..... Last edited by Amber Lamps; 05-07-2008 at 09:27 AM..