Sorry if its already been posted but Asda are selling Mobil 1 5w-50 half price at £17.99 for 4 litres. (well they were at my local branch anyway)

Its the Rally Formula Fully Synthetic Motor Oil. Also used in the SLR-McLaren from the factory so should be pretty good.

 

is 5 - 50 not too thin. i use mobil 1 15 - 50 in mine. crackin price tho for any kinda mobil 1

 

Indeed, stonkin' price, might get some myself.

Bet Mobil are pleased about this, somewhat devalues their high quality product!

Cheers
Simon

 

Question,

What does 15-40 really mean.

I know its the viscosity and came from the time a calbrated ball would sink through a calibrated tube, but what does the variation really, technically, mean

Is it the change in visc with temperature ?, then that sounds like 5-40 is less stable than 15-40 ?

Never found the actual specific answer

 

The first number should really have a 'W' after it to stand for winter and it is the viscosity when cold (not sure of the temp), and the second number is the viscosity when hot. ie 10W-60.

 

? so it gets thicker when it gets hot ?, that don't sound right.

People often state that 15-40 is thicker than 5-40, ie the lower the thinner.

I must be getting the wrong end of the stick

 

And also the difference in the numbers doesn't mean that one oil is better than another. You pick the oil to suit the engine in the climate you live in, in really cold places you would pick a thinner oil when cold so that the car could start easily and the oil is able to flow through the engine.

I use castrol 10W-60 which alot of people on here use, 15W-50 mobil 1 is also popular.

 

The oils are engineered to counter the thinning with increased temperature. They change from a 10 weight oil when cold to a 60 weight oil when hot. They are so called multi-grade.

As you say, if the oil was a straight 40 weight it would get thinner as it warmed, but these are multi-grades.

Is that any clearer?

 

not quite.. it means the hot viscosity is the same as a 50 weight oil, it still thins out but only to the same degree as a straight 50 weight oil.
The xW number is a measure of its cold start charactistics, not directly comparable to an x weight oil.

 

Hopefully reading this will clear up any confusion about how a multigrade works.

In answer to the question here concerning viscosity, yes 15w-40 is thicker than 5w-40 but only when cold, they give the same protection when hot as does a 0w-40.

A 15w-50 is the same viscosity as a 15w-40 when cold but thicker when hot.

It’s written on every can of oil and it’s the most important visible characteristic of an oil. The viscosity of an oil tells you how it reacts in certain circumstances and how it performs as a lubricant.

When a oil is subjected to external forces, it resists flow due to internal molecular friction and viscosity is the measure of that internal friction. Viscosity is also commonly referred to as the measurement of the oils resistance to flow.

There are two methods of viewing an oils resistance to flow. Firstly there is Kinematic Viscosity which is expressed as units indicating the flow of volume over a period of time and this is measured in centistokes (cSt).
An oils viscosity can also be viewed by measured resistance. This is known as Apparent Viscosity and it is measured in centipoises (cP).

However in the real world an oils viscosity is also referred to in such terms as thin, light and low etc. This suggests that the oil flows or circulates more easily. Conversly, terms such as heavy and high etc suggest the fluid has a stronger resistance to flow.

The reason for viscosity being so important is because it is directly related to the oils load-carrying ability - The greater an oils viscosity, the greater the loads that it can withstand. (It must be added when new not over a period of time as all oils “shear down” with use)

An oil must be capable of separating the moving parts in your engine at the operating temperature. On the basis that an oils viscosity is related to its load carrying ability, you could be fooled into thinking that “thicker” oils are better at lubricating but, you’d be wrong in this assumption. The fact is that in the wrong application a high viscosity oil can be just as damaging as using a low viscosity oil.

The use of an oil that’s too “thin” can cause metal-to-metal contact, poor sealing and
increased oil consumption and conversely, an oil that’s too “thick” can cause increased friction, reduced energy efficiency, higher operating temperatures, and poor cold starts in cold temperatures.

It is very important that you select the correct oil, not too “light” or too “heavy” and your Owners Handbook is a very good place to start as it lists the temperatures and options.

Oils thicken at low temperatures and thin as the temperature increases. The actual rate of change is indicated by their viscosity index (this number normally listed on the oils technical data sheet indicates the degree of change in viscosity of an oil within a temperature range, currently 40-100 degrees centigrade)

An oil with a high viscosity index, will normally behave similarly at these two temperatures but an oil with a low viscosity index will behave quite differently. It will become very fluid, thin and pour easily at high temperatures. A higher index is better!

Multi-grade oils are designed to perform at high and low temperatures by adding polymers to a base oil (5w,10w, 15w etc) which are heat sensitive and “uncoil” to maintain the higher viscosity sae 30,40,50 etc. This means that the oil can be used “all year round” rather than using different oils for summer and winter.

It is important to understand that the selection of the correct oil for your car is not just guesswork, you must consider the temperatures at which you need the oil to operate a 0w, 5w oil is better for cold starts as the oil circulates more easily when it’s cold and is able to flow around the engine more easily and quickly, offering protection at the critical moments following cold engine start-up. These oils are also known to give better fuel economy and engine performance.

Finally, all oils “shear” or thin down with use and this means that an oil that started life as a 10w-40 will with use become a 10w-20. The period of time this takes depends on the type and quality of the oil. The most “shear stable” oils are proper Synthetics, either PAO (Poly Alph Olefins) or Esters which have very high thermal stability. They are in general of the more expensive variety but last longer and give the best levels of protection.

Cheers
Simon

 

Wow, you really live up to your pseudonym oilman! Excellent explanation.

 

Thanks, happy to help

Cheers
Simon

 

What a star, cheers. Still sinking in but I think i've got it. (must have 40w-1000 in my brain at the moment)

 

Oilman-What is your oppinion on Mobil 1 5w 50 and 15w 50?

 

Mobil 1 make top quality PAO synthetics, only notch above these are the ester based oils.

5w-50 is an uncommon grade due to the VI improvers required to hold it together, but Mobil do a good one and Fuchs also do a good one. The 15w-50 is another good oil, mainly a track grade due to the 15w as this is not really practical for most road use as they do not provide cold start protection like 10w.

Cheers

Simon.

 

Thanks Simon

 

Its interesting that you say that 15w 50 isnt really practical for road use-most car shops seem to stock loads of the stuff.

 

Just checked my owners manual (MY04 STi type UK).

It states preferred is 5w-30, though 10w-30 and 10w-40 are mentioned.

5W-50 is not covered.

Its strange to see scooby owners mention a whole range of oil grades which are not covered by the manual - maybe Classics have a different oil spec?

 

I thought the mobile one 5w-50 oil was only available in France and other Euopean countries and not the UK?

Thought the Mobil 1 we got in the UK was 0w-40 and 15w-50???

 

Exactly my point, read my posts.

5w-40 or 10w-50 in extreme cases!

I rest my case, why do people ignore the OEM's recommendation, that's all that I can recommend on I'm afraid otherwise I would be unproffessional.

Cheers
Simon

 

I have been lead to believe that Motul 300 15/50 was the bee's knees but now I am not so sure after reading your various threads that 15 is low enough for cold weather. Do you think that this Mobil1 5/50 bargain from Asda is a better all round and all season option for my MY01 WRX? And if I were to stockpile a few cans for the future, does the oil deteriorate in unopened cans after say one or even two years if I were to keep the cans in a garage that gets cold in winter but fairly warm in summer? I could store the cans in an internal cupboard indoors where the temp stays more constant.

 

It may well be although it's too heavy for your car and especially for winter.

The Mobil 1 bargain is once again 15w-50.

I would look at 5w-40 or 10w-50 tops.

Storing oil depends on what it is as the additives tend to drop out over a period although less quickly in "true" synthetics.

Email me for a Silkolene price list.

Cheers
Simon