I just purchased a Volvo 3 bar oil presure guage. Will this work (as opposed to a 5 bar) in a 1991 240 SE? Where can I go to learn how to hook it up? Are the hook ups already in place behind the dash, or am I going to have to run my own wires?

John, 240 SE

It will work but it might max out at times. You will need to wire it. And you will need to have a sender, an adaptor for the block as well as a small elbow maybe. Wire + to a fuse, - to the console ground screw (many black wires there) and the G gets the wire from the sender. The gauge li bulb splices to teh brown instrument wire in the dash.
You need a 1/8" 27 thread female to 14 mm 1.50 thread male adaptor (VDO, Pegasus or Summit Racing). Most senders are 1/8 npt and same with the elbow.

re: "...It will work but it might max out at times...."
Mercedes-Benz owners would all get a laugh out of that.
All M-B's (until late '90's, when they eliminated the oil pressure gauge altogether) have always had oil pressure gauges that normally "pegged" (at 3 Bar) as soon as the rpm went over about 1,000 rpm. I can't tell you how many times new MB owners would write to the MB lists asking if this was normal -- it was completely normal, and a sign of serious problems if it didn't max at 1,500 or more. Moreover, MB owners manuals clearly say that it was acceptable for the pressure to drop as low as 1/3 bar (i.e, a mere 5 psi) at idle, although I never heard from anyone who saw less than 1-1/2 or 2 bar at idle.
Lest I ramble on some more, I think that the point here is that MB, reflected in the specifications of their oil pressure gauges, felt that it was unimportant to know how high the pressure rose, but was very important to know accurately how low the pressure dropped at times. So you should use your gauge without worrying that it couldn't show the maximum pressure -- it's the low end that you want to know about, and the 3-bar gauge can show that more precisely than a 5-bar gauge, viz. the scale on the gauge.

So I too am going to install a 3 BAR gauge in a 1990 240DL - what readings should I expect to get? What's normal at idle and at higher speeds?