It's not the first few minutes of running that matters - it's the first few seconds, when the engine is starved of oil. And you are indeed partly correct in your statement about viscosity modifiers - they are used to "thicken" an oil as it gets hotter, which means that you can run a base oil that is thinner when it is cold!
They are more expensive because they have more additives. Thinnest isn't necessarily best - if the oil is too thin then it will allow metal-metal contact and not do its job. Hence a manufacturer will specify a small range of oil which you can use, or one grade. Obviously once you are within the recommended range, then there is no advantage in running thicker oil - it just decreases efficiency.
Thinnest is often best, when it comes to a cold oil - hence the 0w30 and even 0w25 oils on the market......... you need to get the oil (even thin oil) circulating round the bearings ASAP after the first crank, otherwise you WILL be getting metal to metal contact!
An engine would be "starved of oil" only because its flowed down (gradually over time, due to its viscosity) the upper surfaces back to the sump, after last time it was run hot. And, since 5W/40 and 15W/40 are the same viscosity when hot, they'd have flowed down the same. So there is no advantage here. Perhaps there is a small effect if you ran a car for a few minutes, then switched it off for an hour, then ran it for another few minutes, etc. But the main damage here is 'cold start corrosion' which relates to exhaust gases, not really the oil problem. So its good advice if you switch on your car just to move it, to actually leave it running for 10 minutes and let it warm up. This heats up the exhaust and thus prevents water building up in here, which corrodes it. But I digress...
Agree! So long as its not thinner than the equivalent hot oil viscosity for the manufacturer's recommendation.
Yes, they both flow down the same. But a thinner oil pumps up faster than a thick one, hence the advantage.
so......... a 5W/40 will be thicker when cold (rating of 5 cold) than a 15W/40 (rating of 40 when hot) no sh*t Sherlock but not for the reasons you thought tho
A gentlemanly discussion about the finer points of tribology and engine technology, if you will.......
You tell me the reason I thought so and I'll tell you if what you're thinking is right! I just said it from the beginning, for no other reason than its true.
looks like i missed the fun lol would i therefore gain anything by going fully synth? sounds like ill get protection slightly sooner after start up, i guess itll warm up faster too. what are the differences, protection wise between semi and full synth?
Not a huge amount, on your engine - it'd be different if you were running huge turbo power. TBH if you change your oil often, you'll probably spend more on oil than a secondhand engine would cost you after a couple of years!
I personally wouldn't use any oil less than a 10w in an older higher mileage engine. It's all very well haveing an oil that gets around the engine quickly when cold, but if your'e engine has done loads of miles then it is going to be pretty slack and using an oil that is really thin when cold is just going to cause high oil consumpsion. A good quality semi-synthetic oil is fine for most engines unless, as mentioned before you are running a turbo, or any high spec engine(but just to piont out fully synthetic should not be used for running in).
they were my thoughts also, hence i wasnt too sure on the recommendation. i think ill stick to my millers semi synth. its due a change soon, i need to order a proper 16v sump plug. about 8 from the stealer. such an annoying expense, has to be done tho. http://www.vwspares.co.uk/product_info.php?products_id=2224 just want it cheap and asap!
It's the cold viscosity you need to worry about! especially on high mileage engines, sometimes (always above 200K IMO) even the manufacturers recomendation is too thin. 40 is always OK when hot, it's the 10w and 5w in the first few sec's which give the prob's IM(limited)experience.
A bit rough on the new lad TSC!! Its not what you say sometimes its how you say it (I now know just to ignore you)!! Anyway, whats the best oil for my Mk1 1.8 8v with 196K on the clock??
I'm abrupt. Don't use oil, use bearing grease. It slops around so much less in the corners than a 20/50 would........
