Viscosity range and synthetic oil

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In reading the info on this site that describes the characteristics and processing of oil, it was mentioned that using a multi-viscosity oil with a viscosity range wider than necessary is not a good idea, due to the stress on the additives that are used to make the oil a multi-viscosity product. However, does the same guideline apply to synthetic oil? Castrol makes/made a version of Syntec that was rated at 5W-50; a really wide range. I used this oil in my 1962 Ford Falcon 6-cyl engine and when I would pull out the dipstick when the engine was hot, the oil seemed to have the consistency of water...really thin and "drippy". Not the way I would have expected a 50 weight oil to act under the same conditions. How does the recommendation about viscosity range selection apply to synthetics? What guidelines should be used when selecting a synthetic oil, especially if it will be used in vintage vehicles that were made before synthetics became available?
 
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A synthetic needs less of those additives -- called viscosity index improvers, or VIIs -- to achieve a given spread. For a smaller spread, like 5w-30 or 0w-30, a synthetic usually won't need any VIIs at all. A crude and almost baseless trick I use is to subtract the small number from the big one. If the result is more than 30, it'll probably have VIIs. There's nothing special about the numbering or VIIs that makes that work; I've just noticed that oil formulators that claim never to use VIIs usually make an exception for 5w-40 or bigger spreads. FWIW. So, 5w-50 almost certainly has VIIs. I remember seeing a few UOAs of Castrol Syntec 5w-50 that indicated that the oil had sheared quite a bit under stress, which supports that hypothesis. The recommendation to keep the spread as small as possible still stands with synthetics. The difference is simply that synthetics allow a greater spread with a given content of VIIs. About the oil in your Falcon: All oils get really thin when hot. The differences between a 30 weight and a 50 weight at high temperatures might not be noticeable unless you compare them back-to-back. That said, there is always the possibility of shearing or fuel dilution. As always, oil analysis is a sound practice. to BITOG!
 

Bob_B

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When I used 10W30 conventional oil and I pulled the dipstick on a hot day, the oil would be a thin consistency, but would not usually drip off the stick unless I held it at a steep angle and let the oil run down. With the Syntec 5W50, the oil would run down the stick and drip off, even when held at a shallow angle like 20 degrees or so. It would also be absorbed into a paper towel as quickly as a drop of water, while the 10W30 conventional oil would just sort of ooze into the paper towel fibers. I also started having rod bearing problems after using this stuff for a couple of years. It could have been a coincidence, I suppose, but either way, I don't think that the Syntec 5W50 worked well in the 1962 with 70k+ miles on it.
 
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For a group III synthetic like Syntec, the 5W-50 range is quite wide. For an oil with at least some moderate amounts of PAO Group IV base stock such as Mobil 1 (not sure if Amsoil has oil with that viscosity), that range is not as much of a stretch. But you are correct that it is a pretty big stretch for any motor oil.
 
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 Originally Posted By: Bob_B
When I used 10W30 conventional oil and I pulled the dipstick on a hot day, the oil would be a thin consistency, but would not usually drip off the stick unless I held it at a steep angle and let the oil run down. With the Syntec 5W50, the oil would run down the stick and drip off, even when held at a shallow angle like 20 degrees or so. It would also be absorbed into a paper towel as quickly as a drop of water, while the 10W30 conventional oil would just sort of ooze into the paper towel fibers. I also started having rod bearing problems after using this stuff for a couple of years. It could have been a coincidence, I suppose, but either way, I don't think that the Syntec 5W50 worked well in the 1962 with 70k+ miles on it.
Ah, okay. That's different. The engine was probably shearing the heck out of the 5w-50 then.
 
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