Well it appears that old adage about changing to syn at higher mileage is true. I have a minor leak on my garage floor - i see about a 4"x4" stain every morning when I back out. Nothing major, just kind of funny. Where would these leaks typically come from? Valve cover gasket or what?

The leak could be from anywhere.
Front engine cover, valve cover, oil pan gasket, head gasket (hope not), rear crankshaft seal (requires transmission R&R to fix).
Your best bet is to clean your engine and check where it comes from

I changed to synthetic after nearly 200,000 miles of dino oil, and the small leaks that I had stopped.

No leaks, negligble use.

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Norm in NJ
Next oil change at 230,000miles

I had mixed feelings on synthetic and still personally believe it can increase leaks. ** However **, when I observed Mobile 1 synthetic to stop all chain slap on an SL with new tensioners/rails/etc (apparently the tensioner was bleeding down), I may be sold and now have it in the car. The best lubrication, especially when a car is started cold, would seem to offer many benefits towards the long life of the car. After watching a chain break free and nearly destroy the engine on my SL, I'll do just about anything in the car to never hear that chain slap sound again, however slight.

Check the oil filler cap for leakage. Only happens at speed, can lose quite a bit of oil with only traces on the floor.

Valve cover gaskets need replacing on a regular basis on these cars, they get very hard and dribble. The leak will be worse with Mobil 1 than with dino.

I've not had any real problems with leakage on any of the cars I've swapped over. Once I replaced the oil filler cap on the 300D, I use less than a half quart in 6000 miles. The TE seems to be running about 3000 on a quart these days.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

My 1991 560 SEL has only 104400 miles now. I used casto 10W-40 synthetic on my first change. I used 5Q of Mobil 1 20W-50 and 3 Q of Mobil 1 5W-30 this time. I could smell oil burning now and then. I think I got a leak now at the base of a bolt for valve cover.

Psfred, you are a lucky guy. My car has small leak from timing chain cover, I am in SF Bay area, I use 20w 50 Valvoline (Dealer recommended). My car drinks 1 qt. about 400 miles freeway drive. Don't leave home without a qt. of oil in my trunk. What a life!!!

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89 300 CE

Newkid:

Replace your oil filler cap, it's not the cover that's leaking! Cover only makes a mess, if you are using a quart in 400, I'd bet the oil filler cap doesn't seal.

BigBenz:

Replace both valve cover gaskets. The only time you smell buring oil is when they leak onto the exhaust manifold. They are probably so hard they will break when you take the valve covers off!

Clean the cover and the head with solvent, they must both be completely dry when you install the new gasket. Put gasket on the cover and put cover in place, making sure the gasket is still on (it tends to fall off when you flip the cover over), then lift the cover an inch or so and slam it down on the head. Repeat until it makes a solid "clap" rather than a dull "thud" -- you will hear the difference. Then tighten to 20 INCH lbs on the bolts. No leaks. Any oil on the head or cover, the blasted things will leak immediately!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Swithced my diesel over to synthetic at 234k and no new leaks. The current leaks were unaffected.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Yep, happened to us too. Rear main seal, at 123k miles, very shortly after my first DIY oil change with synthetic.

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1995 MB E320 Wagon
2001 BMW 530i
2007 MINI Cooper S (!)