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A few things:
The faster your engine spins the more oil is pumped by the oil pump. They have a pressure relief valve so they will develop no more than 35 to 45 psi.
However, it is common for an engine to be warmed up and the pressure drops down low to 15 or so at idle. This means the engine has high miles on it, but is not abnormal.
I would suggest change the engine oil and use a heavier grade. Try 10W-40 or 15W40.
Lots of new cars use a thinner oil like 5w30, so the mechanic might have put that in your car in error. A thin oil will build less oil pressure than a thicker oil.
Also, engine oils thin out as they get more miles on the car. Change yours every 3000 km or so. This will help keep the pressure up.
The noise when you downshift is likely from your transmission, and has nothing to do with your engine oil pressure.
As you shift into a lower gear the synchroniser rings in the trans must work to match the speed of the input and output shafts in the transmission. They are likely getting worn out, and require a full trans rebuild to fix.
However, you can get many many more miles out of the trans by making them last longer by double clutching your downshifts.
When you want to downshift, you push down on the clutch, shift into neutral, release the clutch, give the throttle a little poke to get the revs up, push down on the clutch and slide it into the lower gear.
Ideally you want to poke the revs up to the same RPM that the engine will have after you downshift.
Once you practice a bit, it will become second nature to you. People used to double clutch all the time with older cars and most big trucks still need this technique.
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