Interesting Calculation on Oil

So, the other day after I put the car in the garage and let her sit for maybe 5 minutes, I checked the oil on the dipstick and noticed that the level was noticeably higher, yes higher, than when I checked it before taking the car for a spirited run around. I never look at the dipstick when the car has been run, preferring cold. My garage is level. So I ran some numbers on coefficient of thermal expansion of motor oil, the volume in the car, and using a temp of 180 F I found there should be expansion of about .5 litres at 180 F. And sure enough, that was about the increase on the dipstick since cold. Soooo....maybe you guys all knew this but I had never thought of it before. So maybe that's why Porsche recommends taking the oil level when the engine is at temperature because, well, you have more oil....Science is amazing. Rick

Definitely read about it and I think it applies especially during winter months. I'd need to look for it, but I read a technical article that stated that motor oil can expand and contract by as much as 7% between cold and operating temperature. If you take average capacity of 8.5 quarts, the 7% difference will give you about half a quart, which is what you're seeing. I know the gospel around here states otherwise but I've been sticking to measuring when hot about 30 - 45 mins after shutting off the car. The car uses hot oil, not cold, so I want to see the operating temp level.

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It also explains something I noticed recently after I had extensive work done by the dealer on my engine, all preventive maintenance. The work included oil and filter. When the car cooled off the next day after bringing it home, I noticed the electronic gauge showed a click above 50%, but the dipstick was down a hair. So it would appear that the dealer techs purposely filled the oil to a tick down on the dipstick, maybe just using the electronic gauge, or filling by just measuring what they knew she would take. Interesting.

That makes sense. I started my car yesterday and the electronic gauge showed half full. I drove about 50 miles and parked the car in the same place. I came out to grab something from the car about an hour later and the electronic gauge showed 2 bars more than previously. Also, keep in mind that the electronic level sensor, which also doubles as oil temp sensor, is located on the right side of the motor and dipping into the very right of the sump. The dip stick dips into the sump on the left sump. Unless your parking spot is perfectly level there will always be a minor discrepancy. Even a 1 or 2 degrees of slope will make a difference. Also, imagine one of your engine mounts is sagging more than the other - this will affect this too.

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Why are you using 180 F? Isn't the temperature of the oil higher than the coolant temperature which seems to be what you're using in your calculations. I think 200-220 F was normal operating oil temp.

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Good point, but I think it's just misinterpretation on the OP's part. If the car was driven for at least around 30 mins then the oil temp should be right around 200F. Oil can maintain a much higher temperature than water so the 180F applies to engine being up to temp. Since we don't have an oil temp gauge, I use oil pressure as my guide. I know my pressure at idle after a long drive will be 1.25ish BAR. Once I see that reading, I know my oil is up to temp.

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Yes, very good point. So oil "quantity" up another 10% or so. Also there was a post sometime back where a guy recommended not adding make up oil until the electronic gauge was at the bottom, then add no more than 1/2 qt. Of course the objective is not to overfill. This whole subject may have been discussed previously but I didn't see it with the detail of the thermal expansion.

What also matters is the shape of the oil tank. A taller, narrower reservoir, like the dry sump tanks in the air cooled cars have a larger swing in the dipstick versus a wider, shorter tank like our 996 engines. I also think you�ll get swings if you wait longer for the oil to drain from the heads, which are notorious for collecting and pooling more oil than we might think. The earlier cars heads tend to pool more than the later cars due to head design changes.

I'm thinking that a lot more thought went into the electronic oil level gauge the Porsche engineers designed for our cars than just the depth of the oil in the crankcase. I'd like to see the algorithm they use to build the oil level display. I imagine it is not just a simple measurement of the oil level but probably includes corrections for ambient temperature, time since shutdown, oil temperature, maybe coolant temperature, and maybe more. Interesting. Rennlist, helpful as ever, shows a thread below from 2014 which actually covers all this ground. Sorry to drag us through this again. Rick

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Those are some good points too! And don't feel bad about rehashing this. We typically have 3-4 redundant threads talking about 996 values so any technical threads where you can learn something are more than welcome be me!

Another great reason to only fill your oil to the halfway mark on the dipstick and the electronic gauge�🤔

FWIW, one more thought. Just get the UAOS with drainback. It can handle extra oil. That way you can fill it up to the full line on either the dipstick or electronic and not worry about it.