Novice question from a novice oil guy with no mechanical prowess or knowledge--but need advice badly from ppl who know cars and oil!!

I recently inherited a 03 bmw 325i, about 90k miles, car manual recommends 5w30 or 5w40 fully synthetic oil (so far only synthetic oil has been used). What would happen if I added REGULAR 5w30 oil in my car? Will regular oil damage or harm my engine over time. If so, please explain and specify how.

I drive in normal conditions, in a fairly sub-urban area, upstate NY, do not race but do occasional spirited driving.

I have asked several mechanics but all have told me its not a good idea except none of them can explain why or give a clear reason. When I went to the BMW dealer, asking that question to a technician gets them little angry and I got a hurried response, they told me to "do what I like outside at any garage". That was really embarrassing.

To my understanding oil’s main function is to act as a lubricant so why not the use of regular oil in my engine? Or does BMW want consumers buy more expensive materials,be exclusive, that is not completely necessary? As I have read online, in Europe, many car owners do not even use synthetic oil as Americans do but there cars run very long.

Not interested in blending. I also know that synthetic oil is superior to regular oil. I just started working and am paying off student loans and cant afford the $100+ to do an syn.oil change and I dont want to unless I know its completely necessary i.e. damage to the engine, etc.

Advice needed,
Tobin, 23, New York

One of the main reasons BMW wants you to use synthetic oil is that they expect you to run longer intervals... 10-15k miles (if you follow the on-board oil change light). A typical mineral oil will not hold up that long. And if you have to change it 2-3 times as often, then is it really cheaper in the long run?

If you're on a budget, then do your own oil changes - 7 quarts of appropriate synthetic oil plus an oil filter will cost you about $50.


Also, one of the requirements for BMW oil is relatively high HT/HS. You will not find a 5w-30 mineral oil that meets it. High HT/HS ensures an oil film is not broken when you're doing your "occasional spirited driving".


I guess it depends who you talk to. All newer cars in Europe run pretty long OCIs - 20k km or every 2 years. Most mineral oils don't hold up well over such long intervals. I'd be surprised if those owners really run mineral oil in their cars.

My parents who live in Europe have cars that are not that young anymore - a '99 Nissan Primera and an '02 Volvo S40. Both those cars call for synthetic oil, and when they go for service, that is exactly what they get.

I didn't know that.(The HT/HS). Always used syn oil though in the Z4.

If you do it yourself, it will not be $100, and you'll be able to recoup the cost by longer drain intervals, if you use a bmw long-life spec oil.

There are a variety of reasons why dino oils are not sufficient, including the requirement for robust hths viscosity - all bmw spec oils are aces a3, and no decent dino oils in the correct viscosity are.

10w-40 dino would be the closest to reasonable protection available in a dino oil, but given the VII requirements, the oil will shear down quick and need to be changed more often.

You might consider running chevron delo in 15w40, or if you can find it, 10w-30 versions of shell rotella or chevron deli, as either will be more robust. None are optimal, and you'll need to change oil far more often than if going with syn.

I'd pick the right oil - I'd use m1 0w-40, use the oil life monitor to it's fullest extent, and do what is right for the car.

What is HT/HS stand for? and when this film is broken what happens next?

I will definetly have to look into learning to do an oil change if i understand why synthetic oil is required lol

In Europe and Asia I read some BMW owners use regular oil but they do not wait 10 to 15k intervals to do an oil change.

HT/HS - high temp / high shear stability. When the oil film is broken, two pieces of metal come in direct contact with each other - this causes excessive engine wear.

My sister in law tried running dino in her BMW...and with her driving, it really was a big no-no.

I'm guessing you could (she did for nearly 1500 miles-didn't hurt the car), but with the crazy weather in the northeast, and city driving, I would put synthetic in it, even if it had to be wal mart oil...

IMO, a 15W40 HDEO (like Delo) would hold up.

Ohh I see, so your saying my BMW engine gets unusually hot during spirited driving..causing the film to wear out during this type driving?

So does that mean the BMW engine gets hotter than a Nissan sentra if you compared both engines during spirited driving?

I agree with Pete. BTW, how can I find the HTHS of oils?

To JHZR2
You said 10w-40 dino would be the closest to reasonable protection available for REGULAR oil...but your saying it requires more changes because it cannot hold up in shearing.

So is changing the oil every 5k miles a good rule of thumb for this grade of oil in my car?

QOUTE from QAUTTRO PETE--->Also, one of the requirements for BMW oil is relatively high HT/HS. You will not find a 5w-30 mineral oil that meets it. High HT/HS ensures an oil film is not broken when you're doing your "occasional spirited driving".


Ohh I see, so your saying my BMW engine gets unusually hot during spirited driving..causing the film to wear out during this type driving?

So does that mean the BMW engine gets hotter than a Nissan sentra if you compared both engines during spirited driving?

Maybe not necessarily hotter than a Nissan Sentra engine, but BMW engines have typically high output and the construction is such that it puts the oil through some serious paces, so you want an oil that is very stable and still doing its job under those conditions. Then again, I am not a BMW engineer, so I cannot tell you how they determined that an HT/HS of 3.5 is good, but 3.4 is already insufficient.

Forget the HT/HS. Focus on the drain interval - you can go twice as long on synthetic than on mineral, so you're saving time and labor costs. Wouldn't that in itself make a good argument for using synthetic oil?

Are you also not running premium gas in it?

Some companies show it in their literature (PDS) on their websites. Some don't. I have yet to find out HT/HS for the new Castrol Edge stuff.

I kinda think that changing oil every 5k using dino vs every 15k using syn is going to end up costing you more.

Stick with the syn, especially if you are going to do spirited driving.