Mine did that on my last trip to Florida. It would glow bright on start-up then glow dimly until the engine was warmed up. It never does that in Arizona so I decided that it was just Morning Moisture. 
                            Neal 
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Date:    Sat, 24 Aug 2002 14:32:40 -0600
From:    "J. Stefanovich" <jstef@FLASH.NET>
Subject: Oil light

Howdy folks,
        My '82 diesel Caravelle blew a hose last Sunday on I-25 near Las Vegas,=
 NM.  I smelled maple syrup and looked down to see the temp needle creep up=
 over the peg.  I shut it down right away and, after unloading all our=
 camping stuff and seeing what was wrong, called for a tow.  The truck=
 showed up in less than 20 minutes, which was a blessing since I had three=
 kids and three dachshunds on board and it was high desert afternoon hot on=
 the side of the interstate.  We fixed the leak on the spot with a piece of=
 hose from the wrecker's emissions setup and filled the system with water=
 from a tank he carried with him.  I made it back to Albuquerque no=
 problems.  The next day I changed the oil and filter.  No water in the=
 oil.  Now, everything seems fine except the oil pressure light glows=
 faintly all the time.  Comes on bright at start up then fades but never=
 goes out.  Any suggestions as to what might be wrong?  Could the sender be=
 fried?  Any way to check with a multi meter?

Thanks and happy motoring,

Joe