I know this will create a lot of passion on here as oil is so close to people's hearts but what the heck, here goes....

Suppose you have 2 bottles of oil from the same manufacturer and the same model (version!), lets say:

Shell Helix Fully synth.

and both of those are the same quantity, eg 1ltre.

If one bottle was 5-30 and one bottle was 5-40, if you mix them together do you get 5-35?

I am using this thread as self education as I always try to find out what the exact spec oil on a car is and stick to it religiously but I never really understand why there are so many variants. I expect some of it is commercial, ie when I was trying to get the right oil for my missus' motor, I ended up buying direct from LR as all other places never had this magic spec that the manual bleated on about.

Do engines really know what oil is going in them? Suppose there was only one oil available and it went in an engine that wanted another sort - would the engine die?

In a nutshell. WTF is going on with all these oils these days?

 

 

If you have low 5/40 and you chuck in 5/30 that's a whole lot better than NO oil Under those specific circumstances i'd not care a damn what mixed with what. If it was the Aygo, i'd not care under any circumstances at all

 

I've seen the oil wars on this forum in the main section but never really contributed. So I started this one in a peaceful way hoping not to ignite the flame within (pun I spose).

 

Yeah that's what I thought. Might as well top up with some oil of a different sort than be low. I just can't work out what all these types are and why it really matters. If the oils will mix, how the hell can the engine know. Its a lube, its not like its a usb interface that talks a specific protocol.

If you have an oilchange, surely residual will always mix with the new. Its a mindf@#k don't you think?

 

Why not carry out some empirical research; mix the two oils and have the end product analysed to determine its viscosity.

http://www.monition.com/viscosity-testing.html

 

I suppose I could do the same to work out how a nuclear reaction takes place but its far less messy if someone tells me. Wouldn't answer the questions I have posed in relation to why are there so many different types and what makes a type specific to an engine. I could always start building lots of engines but I wouldn't know what needed to be different in order to justify different sorts of oil.

 

Would be similar, the grades are so wide you'll just end up with a thick 30w or a thin 40w though
They are miscable though, so shouldn't be any issues.

 

The thicker the oil the more bearing protection you have but the more drag created meaning you are going to be losing power.

That's the basics. I have seen a car lose a lot of power from the wrong oil in it. This was fixed when the right oil was put in it.

Otherwise, it's temperature resistance - the synthetics resit breaking down better than semi/mineral meaning you can go longer without changing them and run them at hotter temperatures.

One of the most interesting posts on here regarding oil is when Daytona Dave posted three dyno runs with three different oils on the same day without removing the car from the dyno.

The 4k old Petronas did well. Then changed to Castrol Edge 5-40 which lost 4bhp. Then changed to Millers Nanodrive 5-40 which made .7bhp more than the Petronas.

My guess is that the 4000 mile old petronas did well because, having owned that car, the oil became contaminated by fuel and thinned out. The thinner oil meaning less drag.

So what about the Castrol Edge? Looking at the plot it was as if the timing was retarded meaning the knock sensor may have had a part to play. I don't think the loss was friction based alone.

The Nanodrive seemed the best but that is hardly suprising because it was nigh on twice the price of the Castrol edge, and three times the price of the Petronas.

However, as Unclefester says, having any oil is better than no oil. And any decent semi synth will do perfectly fine in 99% of conditions.

 

A few horsepower is normal variation on a dyno. You would have to do a 100 pulls with each oil to see the difference. Not saying there is not but one pull tells nothing.

Oil today if full of different additives. My concern would be the reaction of additives of one oil with additives of the other oil. In short if you are on the side of the road and need oil, put in anything that you have to get down the road if that is what you have to do.

There is just no reason to use your car as a science project to "see what happens" when you start mixing.