I use the port near the distributor on both of my Pontiacs...
X2. Air in the block? New one on me, but I can see the theory of it. So burp the line at the gauge fitting I suppose. Never had any issues with mine. As always, many opinions on this one, but here are a few comments.
Couple of comments from other sites to consider:
RESPONSE No. 1: "There can be quite a bit of difference in the reading between the two different locations. the real reason why you use the hole near the distributor:the pressure at this hole is AFTER the filter and bearings.This allows you to monitor the whole engine.If the filter is going south you'll see it.Bearing issue-you'll see it.Now the reading might be a TAD BIT LOWER than at the filter."
RESPONSE No. 2: "Now that I have my new autometer gauge installed I compared the readings between the stock gauge which is connected to the sender at the oil filter and the new gauge which is connected to the hole next to the distributor. the stock gauge reads 15 pounds higher than the autometer gauge. and that is after having my stock gauge professionally calibrated buy auto instruments in North Carolina. for example at idle my stock gauge reads about 60 pounds but the Autometer gauge reads about 45 pounds. under load the auto meter gauge goes up to around 70 or 80 and the stock gauge eggs past 80. keep in mind I'm running an 80 pound pump in this engine. this test just goes to show that the stock GM gauges are not very accurate.
RESPONSE to above: I don't think the difference is gauge accuracy - or at least not "all" due to gauge accuracy. The port down by the filter has basically "full" system pressure on it. There has been no loss of oil. Oil flows through the filter (couple of PSI pressure drop), then to to main journals (oil escapes out the "ends" and back to the sump), cam journals (similar loss of oil) then to the lifters and out the pushrods to the each rocker arm (more loss of oil). At every point where oil leaks, pressure gets reduced. I think what you are seeing is "real" pressure drop." Simple test - temporarily hook up the Autometer gauge to the port by the filter - no further info showing this test was submitted.
RESPONSE No. 3: "Rocky Rotella responds: Bret, it seems the most common location to plumb a mechanical oil-pressure gauge into a Pontiac V-8 is near the distributor hole on the driver side of the block. There you will find a 31/48-inch-diameter pipe plug with a recessed 51/416-inch square opening. The plug covers a direct path used during initial machining to bore an internal oil passageway that was subsequently tapped and sealed off during production. Because of this and its close proximity to the instrument panel, it makes an ideal location for accurately retrieving an oil-pressure reading.
The pipe plug is commonly removed during engine rebuilds to access the internal oil passageways for cleaning and is typically replaced with a new unit. If your 455ci has been rebuilt in the past, the plug may come out easily. If the engine hasn't, you might find the original plug very tough to break free, possibly rounding out during the attempt. To prevent this, I suggest heating the plug for a short time before hand, it typically makes removal much easier. This assumes, however, that your engine isn't in a vehicle and you can fully access the plug.
If your engine is installed and you don't have plans on removing it, an alternative source for an accurate reading may be the oil-filter adapter. Simply remove the existing electric oil-pressure sending unit, and connect the mechanical gauge's hardware. A drawback to this location, however, is the lengthy distance required to plumb the gauge into the passenger compartment. If going this route, be sure to securely route the gauge's oil line away from the entire exhaust system to prevent burning the line and causing an oil leak."
Response No. 4: "We use the tap-hole near the distributor for more accurate readings as to what is actually going on INSIDE. Rather than at the beginning of the "stream", it's down-river a ways, and will be more realistic."