Mixing synthetic oil and regular oil?

[Rick L]

Is it O.K. to mix synthetic oil and regular oil? I recently brought my BMW into the dealership and they added 1.5 quarts of regular oil to the synthetic oil already in the car. Since the oil change was done recently and not due for a while, is it O.K. to have the mixed oil?

[Hector]

term you reduce the superior antiwear properties of synthetic oil, which therefore, increases engine wear. Synthetic oil is engineered to be stable over a wide temperature range which translates to stable viscosity. You get the same degree of protection when the oil is hot just as when it is cold. For dino oil (or regular oil,) modifiers are added in an attempt to prolong stability over a wide temp range so that when it is cold the oil doesn't get too thick, and when it's hot it doesn't get too thin. Such stability, however, is short-lived.

Dino oil hurts the engine during cold start because the oil is thick and there is hardly any lubrication available which causes engine wear. When the oil is hot it becomes very thin and you get nearly metal-to-metal contact which also causes engine wear.

With synthetic oil, you enjoy it's benefits because the oil is stable over a wide temp range for a long time. You get the same thickness, and therefore same level of protection whether the engine is hot or cold... the engine runs cooler and engine parts don't wear out as quickly.

Having said all this, mixing synthetic with regular oil reduces the effectiveness of the former oil to combat engine wear.

[Rick L]

Thanks for the information... Maybe I should have told them not to add additional engine oil. How many quarts do the 1995 525i take anyway? I did my own oil change with Mobil 1 0W-40 and added 6.5 quarts of oil. The dipstick looked O.K. three weeks ago. I have no leaks what so ever and don’t think I burn any oil either. The Fields BMW service department told me they added almost 2 quarts... WHERE DID MY OIL GO?!?!? Anyway, I’ll take the safe route and change the mixed oil with full synthetic to be sure that there are no conflicts between two different types of oil. I still have an extended warranty for this car that’s about to expire soon and like to keep the car for another two or three years (it only has 75k on the clock).

[Unregistered]

Is it O.K. to mix synthetic oil and regular oil? I recently brought my BMW into the dealership and they added 1.5 quarts of regular oil to the synthetic oil already in the car. Since the oil change was done recently and not due for a while, is it O.K. to have the mixed oil?

Doesn't hurt it at all, don't worry about it. Hector is incorrect about synthetics supposed "superior antiwear properties." Synthetic oils offer 1) Wider weight spans with less VI improvers, and 2) longer change intervals. FYI a 10w oil is a 10w oil at -20f whether it is a synthetic or a mineral oil. If you want superior lube on startup, go to a 5w or a 0w which will be thinner at the same cold temps.

If you want more info on synths, go to bobistheoilguy.com for more info than you will ever want and extended discussions.

[632 Regal]

Heres a quote from Bobs the guys site...

synthetic oils

Oils produced by synthesis (chemical reaction) rather than by extraction or refinement.
Many (but not all) synthetic oils offer immense advantages in terms of high temperature stability and low temperature fluidity, but are more costly than mineral oils.
Major advantage of all synthetic oils their chemical unformity.

[Hector]

you obviously didn't catch my drift. Read the post again, and again...

Hector is incorrect about synthetics supposed "superior antiwear properties." Synthetic oils offer 1) Wider weight spans with less VI improvers, and 2) longer change intervals. FYI a 10w oil is a 10w oil at -20f whether it is a synthetic or a mineral oil. If you want superior lube on startup, go to a 5w or a 0w which will be thinner at the same cold temps.

If you want more info on synths, go to bobistheoilguy.com for more info than you will ever want and extended discussions.

[Bill R.]

10w oil is a 10w oil at -20f regardless of whether its synthetic or not..Thats not true.. The lower number on oil viscosities is arrived a 0 degrees F and many 10w dinos are not 10w anymore at -20 whereas most if not all synthetic undergo very little viscosity change from 0 to -20

you obviously didn't catch my drift. Read the post again, and again...

[Unregistered]

10w oil is a 10w oil at -20f regardless of whether its synthetic or not..Thats not true.. The lower number on oil viscosities is arrived a 0 degrees F and many 10w dinos are not 10w anymore at -20 whereas most if not all synthetic undergo very little viscosity change from 0 to -20

Uh, sorry but wrong. The lower weight number is not tested at 0F. In fact it depends on weight. It breaks down as follows:

0W 3250 @ -30C
5W 3500 @ -25C
10W 3500 @ -20C
15W 3500 @ -15C
20W 4500 @ -10C
25W 6000 @ -5C

The thickeness measurement is viscosity in Centastokes. So when you look at an oil that is rated as 10w it doesn't matter if it is synthetic or Mineral, it has to be less than 3500 centastokes at -20C. If it was significantly thinner at that tempurature it would be thin enough to be called a 5w.

The advantage of synthetics at cold temps is that you can formulate 0w oil which requires so many VI improvers with a Mineral Group I or II oil (we will for purposes of this discussion consider Group III hydrocracked base stocks to be synthetic, even though that is debatable) that the oil would not be functional and break down in short order. Because synthetics also have a higher VI (viscosity index) than mineral oils you can have a wide weight span that mineral oil could not (same reason of to many VI improvers in the mineral). So Synthetics allow for you to purchase oils like 0w-40 or 5w-40 which cover all temps.

Finally, even synthetics undergo significant thickness changes from 0f to -20F. If you would like to e-mail me, I would be happy to provide the PDF charts from Mobil showing the thickness of their various oils accross all tempurature ranges. The oils whether synthetic or mineral thicken at the nearly the same rate with only minor differences in the slope of the thickness curve between the two types of oil (note the graph shows a straight line, but it is in fact a logrithmic scale).

Here is some more info on measuring the thickness of oil:

http://www.wearcheck.com/literature/techdoc/WZA007.htm


Cary@ifn.net

[Bill R.]

read that the viscosity ratings for oils were arrived at 100 C and at 0 C for
w oils... but everything i find states the same as what you're saying.

Uh, sorry but wrong. The lower weight number is not tested at 0F. In fact it depends on weight. It breaks down as follows:

0W 3250 @ -30C
5W 3500 @ -25C
10W 3500 @ -20C
15W 3500 @ -15C
20W 4500 @ -10C
25W 6000 @ -5C

The thickeness measurement is viscosity in Centastokes. So when you look at an oil that is rated as 10w it doesn't matter if it is synthetic or Mineral, it has to be less than 3500 centastokes at -20C. If it was significantly thinner at that tempurature it would be thin enough to be called a 5w.

The advantage of synthetics at cold temps is that you can formulate 0w oil which requires so many VI improvers with a Mineral Group I or II oil (we will for purposes of this discussion consider Group III hydrocracked base stocks to be synthetic, even though that is debatable) that the oil would not be functional and break down in short order. Because synthetics also have a higher VI (viscosity index) than mineral oils you can have a wide weight span that mineral oil could not (same reason of to many VI improvers in the mineral). So Synthetics allow for you to purchase oils like 0w-40 or 5w-40 which cover all temps.

Finally, even synthetics undergo significant thickness changes from 0f to -20F. If you would like to e-mail me, I would be happy to provide the PDF charts from Mobil showing the thickness of their various oils accross all tempurature ranges. The oils whether synthetic or mineral thicken at the nearly the same rate with only minor differences in the slope of the thickness curve between the two types of oil (note the graph shows a straight line, but it is in fact a logrithmic scale).

Here is some more info on measuring the thickness of oil:

http://www.wearcheck.com/literature/techdoc/WZA007.htm


Cary@ifn.net

[ryan roopnarine]

bill, whilst you're a captive audience........you live in arizona so i assume that your car is exposed to the warmer environs. i've heard sooooo many people say that i shouldn't be running m1 5w30 in my 40k-50k mile m50...right now temp is around 85, and by summer should be unbearably hot here....should i be using the 15w50 or 10w30 instead?