[v8volvo]

Engines with EGR (1985+ US D24T and some 1984) tend to build up intake sludge due to the soot in the recirculated exhaust mixing with oil from CCV gases. The EGR valve is on the turbo charge pipe which enters the intake manifold between cyls 2 and 3, so airflow to the rear cylinders may determine why yours showed the most plugging there. However, I have seen them with a significant amount of buildup in the front end as well. Often on the EGR motors you can pull off the turbo blowoff valve from the front of the intake and find it completely packed in.

It's not a serious problem, as you pointed out, but preventing the buildup is as simple as disabling EGR by removing the vacuum line from the EGR valve. If you want to really cover yourself you can make a blockoff plate as well. The EGR control system on these old mechanical engines is primitive and works in a crude way even when everything is brand new and operating to spec -- it relies on various temperature-actuated vacuum switches, throttle switches, modulators, etc and depending on what kind of shape that is all in, just as likely as not that by this point it's running EGR when it shouldn't and vice versa. You're better off with out it, unless you want to spend the time to fully check it all over and ensure it works as designed and then deal with cleaning the intake every so often.