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When i newly bought the engine, i was using synthetic oil. But the oil is always going down. So my mechanic adviced me to stop using synthetic, that the engine is getting weaker and burning oil. He said it's a waste of money, that i should use mineral oil and service it regularly (monthly thereabout)
 

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I'm glad I'm not the only one.....my son was wise enough to stop the car and call when the STOP light came on (complete with coolant vapor coming out the vents inside, after the heater core burst). I was able to nurse the car home that time since it was about 10 miles away and 5000 ft up. Then my wife was driving it when the STOP light came on but thought 'it's only a few miles home'. That time it required a new head. This was a heating issue and not oil pressure and was fixed by a new radiator. But yeah, don't ignore that STOP light unless you're sure it's a bad sensor, and it doesn't sound like it is in this case.
 

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When i newly bought the engine, i was using synthetic oil. But the oil is always going down. So my mechanic adviced me to stop using synthetic, that the engine is getting weaker and burning oil. He said it's a waste of money, that i should use mineral oil and service it regularly (monthly thereabout)
Synthetic oil isn't really optional on the 1.8T engine. Your mechanic gave you bad advice. If you only did that for a short time, I doubt that the problem is all caused by you using (a good grade of ) regular oil. My 2001 ran about 35k miles on conventional oil when new, and it still has good oil pressure at 165k miles, although it did start using a bit of oil between changes, starting at 80k miles, and gradually increased to using 1 quart per 1000 miles (~ 1 liter per 1700km).

If I recall correctly, the oil pressure/stop engine warning only comes on when the RPM is above 2000. Oil pressure increases with RPM, so if your engine's condition is marginal it might beep at 2200 rpm, and not beep at 2800 rpm. Oil also gets much thinner when fully heated up, which causes indicated pressure to drop. My idle pressure might be 60+psi at cold startup, but is 22psi when fully warm.

The pressure alarm is activated when the pressure is below approximately 1/2 of the minimum normal pressure specification, so things are already quite bad before you get a warning at all.

-Gary
 

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Please note that changing the type or weight of oil will NOT "solve" an oil pressure issue. The serious problem remains; you've simply pushed it just past where the warning goes off. And arguably, the warning comes way too late on these engines...
 

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that the engine is getting weaker and burning oil.
are these not all signs of a blown/leaking head gasket?
misfire codes perhaps soon to follow?

Off Topic:
don't ignore that STOP light unless you're sure it's a bad sensor,
uh...and just because it doesn't say stop doesn't mean there isn't a problem. On my adopted 02 GLX 4mo 2.8L wagon, there was so much "chalk" in the coolant reservoir, that even if it was bone dry, it wouldn't notice and it wouldn't alarm.

hopefully after the new year I will be able to take off the intake manifold and replace the gasket that I need to solve my coolant leak.
 

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"weaker and burning more oil" , considering the marginal servicing it seems to have had , poor advice that has been issued, and the fact it is a 1.8, I am leaning towards the PCV system being clogged to the point of not being effective at all.
 

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A clogged PCV system can cause increased pressure inside the engine and lead to leaking valve cover gaskets. If enough oil leaks out the VC gasket it can pool in the spark plug wells and cause mis-fires. Do you have oil in the plug wells?
 
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