Vintage Cars: 72 350 SL oil pressure and squeek sound from rear axle, high pitch sound, sl 107

Question
Hi Ron
I purchased a pristine 350 SL 107.044 from 3/72 with 93 k miles last year from the US and imported it to Denmark. You can see it here: http://www.classicmotorwerks.com/72mb350sl.html
It has been running like a dream since. However there is a squeaky sound from the rear axle or left rear wheel. It is a high pitch sound (not constant) that comes with every turn of the rear wheel. It goes away when I apply the brakes. I have changed the rear axle oil with Castrol SAE 90 but is is still there. I have been told that it probably is the rear diff or a brake issue. Do you have an idea ?  
Also I changed the engine oil and filter using Hengst filter and Castrol classic oil 20W/50 - the old filter that was in it was not an approved make and it said "made in India" which worried me. The old oil seemed very thin but not too dirty. The oil pressure has since gone up when the engine is idling between 750 and 800 rpm. It now reads approx. 35 on the gauge and max. (45) as soon I rev it. Before the oil change it was 30 when idling and 45 as soon as I reved it. The new oil is still clean after approx 150 miles which should be a good sign for engine condition. Is the increased oil pressure a problem ?
Thanks a lot for your input.
I enjoy reading your very useful advice.
Kind regards

Kasper

Answer
Hello Kasper, that is indeed a fine machine, I actually look at that site quite often and lust over the cars.
Mine is just like yours, except I do not have any of the power options , my windows are the crank style!.  Mine is actually an early  72 model, Number 500 off the line. I rebuilt it myself, I am about done except I am looking for a set of euro lights for the front to replace the bugeyes up there.
http://img.villagephotos.com/p/2005-11/1113866/1992331
If you ever come across some in Denmark , let me know, they are rare here in the states and very expensive.
Now onto the issues- first off improved oil pressure is a good thing, low is bad, I suspect that you now have the proper filter and oil weight the pressure is where it should be.  No worries there.
The squeek from the back can be as simple as a brake shoe that is rubbing on the rotor, if it goes away when you brake that is most likely the issue.  The rear brake pads have wear sensors,so if the pads are getting thin , they actually are designed to squeek.  Pull a wheel and check pad thickness, that may be your issue.  THe other "thing" that squeeks back there is the flex disc.  That is the rubber disc that attached the drive shaft to the differential, it has 8 or so bolts going through it.  Look carefully there for cracks or missing/loose bolts and repair if needed.  Let me know what you find and we can go from there.  My guess is that it is brake pad realated.  These are fun cars, enjoy the ride