oslouie

My 04 525i has a small oil leak (which I plan to fix but can't at the moment). When I bought the vehicle, I think the place I bought it from used dino oil because after I changed my oil to Mobile 1 0w40, I would notice an oil burning smell. I think this is due to using a synthetic, causing it to leak a little more than the conventional oil would. Which conventional oil can I use and be safe with?

Margal

I am not expert on oils but you should not use. Conventional or non synthetic oils on these cars. OW-40 is thinner oil and that's why you see more leeks. I only using 5w-30 all seasons as recommended. Many are using 0w-40 here but I afraid that in old engines it may be leeking more.

BimmerFan52

As the Italians say in New York "Fugetaboutit" !

There is NO good reason to move to conventional petroleum oil. It is an old wive's tale that moving from conventional motor oil to synthetic motor oil or visa versa causes engine gaskets in motor engines to leak.

The BMW engine and its gaskets were designed for synthetic motor oil. The engine is filled and tested at the factory with synthetic motor oil. A change to conventional motor oil will not affect gasket swell, and therefore will not cause engine leaks. BMW goes so far as to issue a list of approved motor oils for BMW engines. For your engine, BMW list LL-01 contains several excellent Group IV full synthetic motor oils, including Mobil 1 0W-40.

Conventional petroleum motor oil (comes from the ground) has gotten MUCH better in recent years to the point that it is very difficult to discern the differences between certain well made petroleum oils (Group III heavily hydroprocessed oils) and a Group IV, full synthetic motor oils. However, these differences are small until the oil begins to age and extreme heat and stress is added to the mix. Then the Group III oils are quickly passed in performance by Group IV full synthetic oils.

Synthetic motor oil really shines in cold weather because it flows very well (begins lubricating the critical parts of the engine) very quickly at extremely low temperatures. Of all of the places I could pick in the continental US where synthetic oil is MOST important for cold starting in the winter (called the low temperature pour point), Minnesota would be the very top of my list.

Your leak will NOT be fixed by changing to a conventional petroleum motor oil. Your engine could see extensive wear during starting in very cold weather by changing to conventional oil because it doesn't flow immediately and lubrication is delayed.

And your leak will not be fixed by changing grade of motor oil. A 0W oil will not tend to leak through gaskets more than a 5W oil.

The choice between 5W-30 Group IV synthetic and 0W-40 Group IV synthetic is basically personal preference as both will provide excellent protection in nearly all environments.

The Society of Automotive Engineers (SAE) defines a numerical system for grading motor oils according to viscosity (a measure of flow). The suffixes (0, 5, 10, 15 and 25) followed by the letter W designate the engine oil's "winter" grade, and the next number is the summer grade. These numbers are determined by measuring the kinematic viscosity of the oil at two different temperatures. Each potential number has a range.

There is very little difference between a 0W and a 5W. Because each is in a range, if a given 0W oil happen to fall at the high end of the 0W range, and the 5W oil happens to fall at the low end of the 5W range, both will be on the border between the two grades, and are for all intents and purposes the same viscosity. The kinematic viscosity is a measure of flow, and is not a direct physical relationship to "thinness".

My advise: Continue to use Mobil 1 0W-40, check your oil level often, and fix your leak when you can.

Margal

Thank you for educating us on this subject that is truly great info. We all are learning through experience, practice and reading. In my experience when I switched from 5W to 0W in my privous 325i with 150k miles and which had some oil leaks after switching to Ow I saw more oil leaks. I am not saying that it couses leaks but it seeps easily through already bad gaskets and seals. That why I prefer to stay with 5w-30 This is my opinion on 0w but in theory it is thinner oil and gets even thinner when it get hot. Which is good at cold winter morning start ups

BimmerFan52

I understand what you are saying, and if on your 325i after going from 5W to 0W and having experienced more leaks, you then went from the 0W back to 5W you had less leaks, your emperical evidence would suggested that 0W leaked more. You attribute that to the Winter number and the viscosity of the oil.

However, in a multi-viscosity oil, the W number (winter grade) is meaningless when considering the operating temperature of the oil. The W number is indicative of the cold pour point, which is a measure of how the oil flows at extremely low temperatures. Because the synthetic base stock has no paraffinic content (waxes naturally present in oil from the ground) it does not stiffen up as much as a petroleum based oil does at cold temperatures and begins lubricating almost immediately. The lower the W number the better in cold climates like Minnesota.

In the case of operating temperature, the second number, after the dash is the number to examine. A higher second number (summer number), means the viscosity of the oil will be higher at engine operating temperatures when a gasket will tend to leak more.

If the oil in question, Mobil 1, 0W-40 oil with a summer number of 40, is compared to another 5W-30 oil with a summer number of 30, the 0W-40 will have a higher viscosity at the normal operating temperature of the oil and theoretically should leak LESS.

In reality, there is little attention paid to oil grade based upon the potential for leaks as there is no relationship.

The winter grade is very important for cold starting and the summer grade can be very important in certain engines where oil temperatures are very high (engines with turbo chargers or engines with piston squirters where oil is sprayed on pistons to cool them, resulting in significant elevations of oil temperatures).

If the oil viscosity drops too low and there is high shearing of the oil molecules due to heavy engine load (extended high RPMs or towing for example) the viscosity can reach a point where the hydrodynamic film used to separate key engine components such as journal bearings begins to break down and turn into a barrier film where metal makes contact with metal, resulting in excessive component wear and potentially eventual engine failure.

The oil industry and engine manufacturers have a specific test for oils to measure the oil while at 150C and high shear, with the results expressed as a HT/HS or high temperature, high shear value. Generally a higher viscosity summer number relates to a higher (or better) HT/HS number. The Mobil 1 0W-40 exceeds the minimum HT/HS number most engine manufacturers look for. Some 5W-30 oils will also meet the minimum HT/HS number.

In conclusion, the Mobil 1 0W-40 oil is great for cold temperature starting, is great for high temperature protection of the engine, and if you want to believe (even though it is not true) that a kinematic viscosity will change the tendency for a gasket to leak the higher summer value of the Mobil 1 0w_40 will provide that protection too.

As icing on the cake, the Mobil 1 0W-40 is now available at most Walmart stores, or on-line with free shipping to any store for $22.47 for a 5 quart jug, or less than $4.50 per quart. One of the bargains of the century.