Don't change your oil!

[94 formula]

I change my Mobil 1 every 5,000 miles.

I do have an idea as to why modern motors can go so long between changes and last longer in general. I think electronic port fuel injection is the biggest factor. here is an example:

With a carbed motor on the intake stroke the cylinder is filled with a mixture of air and fuel. This fuel washes the oil off the cylinder walls and gets in the crankcase contaminating the oil there as well. As the piston comes up for compression the rings experience heavy wear due to the fuel soaked cylinder walls lacking some lubrication. As a result the engine wears out more quickly and the oil gets contaminated faster.

With a port FI motor on the intake stroke the cylinder is filled with nothing but air. There is no fuel present to wash the oil from the cylinder walls so they stay well lubricated. Then the fuel is quickly injected near top dead center just before the spark ignites the mixture. It immediately burns and there is much less chance for the fuel to contaminate the oil.

I'm sorry but I have to disagree on this one Trace. What about 93 LT1's that use batch fire? They have the fuel injected and it lays infront of the valve and is sucked in when it opens and there doesn't seem to be a shorter life for them. Also I'm pretty sure at TDC the vavles should be closed and fuel cannot be injected. Fuel is injected on the intake stroke as the piston travels down and pulls in air the injector fires gas into the cylinder. Diesel engines do use special injectors and inject the fuel right before the piston is at the top of its stroke.

[TraceZ]

I'm sorry but I have to disagree on this one Trace. What about 93 LT1's that use batch fire? They have the fuel injected and it lays infront of the valve and is sucked in when it opens and there doesn't seem to be a shorter life for them. Also I'm pretty sure at TDC the vavles should be closed and fuel cannot be injected. Fuel is injected on the intake stroke as the piston travels down and pulls in air the injector fires gas into the cylinder. Diesel engines do use special injectors and inject the fuel right before the piston is at the top of its stroke.

I see your point. at TDC the valves would be closed. The fuel would have to be shot in while the valve is open during the intake stroke. I had neglected to figure in the valve between the injector and the cylinder.

[94 formula]

Your theory may hold some water though when it comes to diesles though. I don't have a whole lot of experience with them except on farm equiptment but I have heard many guys say they seem to just be getting broken in around 100k miles.

Also heres another question: I've heard many different people talk about fuel washing down the cylinder walls but how does that happen? Don't the rings wipe the oil up then back down and create a seal? If gas was able to wipe the walls down couldn't it get in the oil then or wouldn't the oil be burnt up if it wasn't wiped down by the gas? I've just always thought the rings created a seal that kept the gas and air above the piston and the oil below.

[Kevin - Blown 95 TA]

The fuel getting into the oil is minimized in modern engines as compared to older carbed engines, but still, if you use your car for short trips around town, or drive it in winter, it has to run a little rich in open loop for 3 minutes or so until it warms up and the O2 sensors heat up & it goes into closed loop where it runs more efficiently and all the clearances tighten up. And this is on a car that is running good with everything working the way it should. So some fuel is going to get in there, regardless. Some of that will be burned off by PVC system, especially if the car gets some longer trips, but not all of it. Also, some of the combustion gasses escape by the rings and end up in the crankcase - these gasses and combustion byproducts contain acids - the same ones that dissolve mufflers and marble statues (smog), etc, and thats primarily what you want to get out. Does all this mean that if you change synthetic at 5k intervals that your engine will be worse off than someone who changes at 3k - might be tough to prove. I think a lot of us change at 3k just because it gives us confidence that the oil is cleaner. Anyone here ever change his oil at 3k and NOT had it pour out all black & thinned out? Doesn't mean the oil's life is over, but it is definitely different than when it went in, No?

[wolfdogs]

Ken,

Is that the change interval the fleet is using for the filter as well?

yup. and we have over 201,000 vehicles now. very few oil related engine rebuilds.

[Trash man]

DPFI aka direct port fuel injection, they are starting to experiment with that now from what i heard. same thing as a diesel

[Craig 94 TA GT]

Anyone can do an oil analysis test for pretty cheap (~$20) to test how well their oil does at specific intervals. Patman on cz28.com went over 6000 miles with German Castrol Syntec, and the wear levels were amazingly low. http://web.camaross.com/forums/showt...t=oil+analysis

[94 formula]

The fuel getting into the oil is minimized in modern engines as compared to older carbed engines, but still, if you use your car for short trips around town, or drive it in winter, it has to run a little rich in open loop for 3 minutes or so until it warms up and the O2 sensors heat up & it goes into closed loop where it runs more efficiently and all the clearances tighten up. And this is on a car that is running good with everything working the way it should. So some fuel is going to get in there, regardless. Some of that will be burned off by PVC system, especially if the car gets some longer trips, but not all of it. Also, some of the combustion gasses escape by the rings and end up in the crankcase - these gasses and combustion byproducts contain acids - the same ones that dissolve mufflers and marble statues (smog), etc, and thats primarily what you want to get out. Does all this mean that if you change synthetic at 5k intervals that your engine will be worse off than someone who changes at 3k - might be tough to prove. I think a lot of us change at 3k just because it gives us confidence that the oil is cleaner. Anyone here ever change his oil at 3k and NOT had it pour out all black & thinned out? Doesn't mean the oil's life is over, but it is definitely different than when it went in, No?

So its not really the fuel that gets into your oil but more the gasses after combustion that still contain small amounts of fuel? Is it pushed in because of the pressure of the detonation, and is the reason oil is isn't burnt up because it does not have pressure pushing up on it like the gasses do pushing down on them after conbustion? I'm sorry for all the questions but I'm just starting to really understand all this and am trying to soak up all the knowledge I can.

[bigtow]

I see your point. at TDC the valves would be closed. The fuel would have to be shot in while the valve is open during the intake stroke. I had neglected to figure in the valve between the injector and the cylinder.

there is one excption to this trace and co. mitibuishi have G D I gasoline direct injection and this system works in a simaler way to a diesel sorry to be anal about such things but stuff like that gets stuck in my head. i just wish i could put some ot the theory into practise

[Screaming mad]

Help! u've all made me paranoid! All I here is mobile 1 ! I change my oil and filter every 5000 miles (ish)! But my 1994 camaro 3.4l v6 is using good ol mineral oil, 15- 40 castrol designed for high milage engines... I spoke to my local american garage ( I live in the UK ) and they informed me to stick with what the engine already has in it, as swopping oils can cause damage, damaged seals etc etc, and to just use a decent oil, and change every 5000 miles! Any advice or views apreciated

[Steel2686]

Help! u've all made me paranoid! All I here is mobile 1 ! I change my oil and filter every 5000 miles (ish)! But my 1994 camaro 3.4l v6 is using good ol mineral oil, 15- 40 castrol designed for high milage engines... I spoke to my local american garage ( I live in the UK ) and they informed me to stick with what the engine already has in it, as swopping oils can cause damage, damaged seals etc etc, and to just use a decent oil, and change every 5000 miles! Any advice or views apreciated

This is always a debated topic. It's tough to really tell if switching oils could hurt the engine in any way, but switching to a full synthetic is never a bad thing necessarily. I've heard only the opposite - If you've been using a full synthetic like Mobil 1, you shouldn't switch back to conventional oil. This could just be an urban legend. Maybe Mythbusters could tackle this one.