On a 1989 420, using 5w50 synthetic, I noticed that when the engine temperature is 100 or more, usually after heavy stop and go city driving, while at a stop light in 'drive' the rpms are at 500, and the oil gauge indicates 0.5 bar. If I slip it into neutral, rpm goes to about 550-600, and oil gauge reads 1 bar. Is this normal?

When the temp. is around 80, at a stop, and in 'drive', the oil guage shows 1.5 bar. On start-up and speeds above 20, the oil gauge reads 3 bars.

The oil and filter change was about 1,500 miles ago. The filter was changed a second time after 1,200 miles, I thought the synthetic would remove a lot of dino crud since this is the first synthetic oil change. The oil is at the proper level.

I feel the oil guage should never show less than 1 bar so I'm concerned about 0.5 bar at a 500 rpm hot idle. What should I replace or check?

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1989 420 SEL