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post #18 of (permalink) Old 26th February 2007
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Its fair to say that historically there were problems with synthetic oil in old/ high mileage engines. The main problem was that the old seals had already failed / cracked and that although weeping oil most was held back inside the engine by the hardened gunge that was stuck on components by the seal; effectively partially filling the gaps in the failed seal. The problem then became that the newly introduced synthetic oil , either following a flush or otherwise, would over time erode and carry away all the gunge and varnishes in the old engine. When this happened oil would again be able to get past the failed seals and at an ever increasing rate. The synthetic oil had not caused the seal to fail, it had failed long ago; but the synthetic oil had revealed that failure and in cleaning inside the engine caused oil to escape past the seal. This situation was not helped by a couple of synthetic oils with poor formulation actually causing some old style seals to shrink!
All this was in the 70's, anything in the last 20 years or so won't have the latter problem as all engine manufacturers and all oil companies make compatible products and have done so for decades if they had those problems in the 1st place.
The next point is about putting synthetic oil in a 'high mileage' engine. Well forget the high mileage bit just use the term knackered and you will understand the problem. No top quality oil (not even Amsoil) can put metal back. If an engine has been thrashed to death and is on the point of expiring then putting in a synthetic and thinking you can go racing will have but one result. And of cause its the oils fault? Modern engines (ie from the 80's onwards) have already benefited from better lubrication (even if it was just cheap mineral oil ) have benefited from better components with better tolerances made of superior materials. Whilst my mothers MK1 Escort expired at 80,000 miles with a worn out engine (remember those days?) cars should be expected to easily exceed double , triple or much more than that. The important thing is the mechanical condition. A sound engine of whatever mileage will always benefit from a better oil . As long as the 2nd number of the multi grade is the same as it was with the previous mineral oil it will 'float' the rods and bearings in a similar fashion as it has after all been graded in the same manner. What would be different would be that under strain the synthetic oil will not be as likely to shear (break) and some just wont shear at all. The synthetic oils being slipperier may flow better but this will merely remove heat better which is good ; this can on the odd occasion (when pumps are worn or bypass valves weak) be seen with a very slight drop in oil pressure. Nothing to worry about as the synthetic oil is between 3 and 5 times more robust (film strength) and that is when the oils are new. The difference is even more marked when the mineral oil gets old and has sheared back. At this point you wouldn't notice a difference in these odd worn engines as the 'sheared back' mineral oil would now be thinner than the unsheared or unshearable synthetic oil which would now be showing a better oil pressure than the old mineral oil .
When does a mineral oil start to shear back? Its actually from the time the petrol air mixture or compressed diesel first ignites!
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Last edited by amsoil; 26th February 2007 at 07:23 PM.
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