Hello!
Today I checked the oil level on my 02 Saturn SL2 and found it was very low! Under the low mark in fact. I looked around for leaks but couldn’t find any. Removed the oil cap and found sludge.
I changed the oil 1000 miles ago with full synthetic. I also replaced the PCV valve not long ago. Could it be a bad or incompatible valve?
I filled it with oil again but decided not to drive it atm. The car drives fine, no funny noises or anything.
Please advise. Thanks!
The engine is probably burning the oil.
Did you know that some auto manufacturers consider 1 qt/1000 miles acceptable oil consumption for their new cars?
Just get in the habit of checking the oil level every couple of weeks.
Tester
How many miles on this old Saturn? 100,000? 200,000? Get used to checking the oil at every gas fill-up.
Apparently according to one of your other threads the vehicle had been doing fine with conventional oil.
Also you should at least check the oil level once a week. You do have a 17 year old car.
I even check our new car once a week.
I didn’t know this 
I have never seen this before in the cars I previously owned. The car takes 4qts. How do you know how much it burned or how much to add? The oil level was below the low mark. I thought 3 qts were gone in 1000 miles!
It has 167,000 miles now.
Add oil until it reaches the FULL mark. If it is at the low mark, 1/2 to 1 quart added will top it up. Do not overfill. Check with the engine OFF on level ground.
You mean you’ve owned other cars and NEVER learned to check your oil?
So it is not the PCV then? 
Okay I will check the oil more frequently now.
Monitor the oil level until it’s one quart low. Then add one quart.
Tester
Check your Owner’s Manual for directions.
Otherwise, there’s probably a quart’s difference between the low mark and full mark. Start by adding a quart. After several hours check again and see where it stands. Add oil in smaller volumes until it is brought up to full. It’s not good to over-fill it by much.
CSA
With my previous vehicles, I just took them to a shop for regular maintenance. From time I would check the oil myself, but no I didn’t know you have to check it that often, unless you see leaks and what not. That’s what I thought. Sorry for my ignorance.
Ohhhhhh! got it 
Thanks CSA.
No need to apologize. Ignorance can be cured. 
Small oil pans with only 4 quarts of oil. 5000 miles between oil changes. Even a car that only uses a quart every 3000 miles will be a quart and a half low at the next change. A car with 167,000 miles will use more, much more. Just check it more often and add as needed. Ignorance solved! 
Alright! will do! 
What weight was the synthetic oil?? Today’s synthetics are usually 0W20, an extremely thin oil with great film strength that give high miles per gallon and good low temperature starting.
Unfortunately your Saturn was designed likely with looser clearances and for 5W30 mineral oil. Many older cars start “using oil” when switching to synthetic.
That’s why now car manufacturers state in the owner’s manual that 1 quart per 1000 miles is considered “acceptable”. I rented a Toyota a few years ago and the statement was there all right. The engine used 0W20 synthetic and Toyota was aware that under very hot conditions this engine would use oil.
These things are all done to meet Obama fuel consumption standards.
So, just check the oil more often and add as needed. Switching back to 5W30 mineral (non-synthetic) oil may slow the oil consumption.
My wife’s Mazda 3 was designed to use thin oil and experiences no consumption during the 5000 mile drain interval.
I used the 5W30 Mobil1 Extended Performance. It wasn’t burning any oil before so I guess I am going back to the regular oil. 
It could be the pcv system’s involved with the oil loss. But seems pretty unlikely. You could ask your shop to do the basic pcv test, usually means to pull the pcv valve and its hose out of its rubber grommet in the valve cover area and determine if you can feel suction on the end of the valve not attached to the hose when the engine is idling. If the pcv valve itself is faulty it can cause increase crankcase pressure and blow oil seals, but that usually causes a visible oil leak that drips onto the ground. There’s been a few posts here of posters switching to synthetic and experiencing a more rapid loss of oil than they had using conventional oil. You might could find some of those threads using the forum search feature, that icon that looks like a magnifying glass upper right. Best of luck.
I would certainly go back to conventional oil for an oil burner and would buy the cheapest oil that met the specs. On a national level that would probably be Walmart’s Supertech 5W30 in the 5 Qt jug unless you find a good rebate for something else.
Every car I have owned in my long life had the low mark on the dipstick at 1 quart low. I was surprised the my current 2012 Camry has the low mark at 1 1/2 quarts low. I would not be comfortable letting it get that low.
One of my cars consumes a quart of oil every 1000 miles, it hasn’t changed in the last 7 years. I check the oil level every other month, maybe after I retire I’ll check my oil every Saturday.
I have a suspicion that the oil consumption rate of this Saturn has not been closely monitored during the last 17 years, I doubt that synthetic oil made any difference.
So he invented CAFE?
Up until 2009 we were all driving 12MPG tanks with impunity??