Hi @Simon D if we work backwards from the problem, then you may derive your own conclusion here:
So the problem starts with engines equipped with EGR (Exhaust Gas Recirculation), where some of the exhaust gas containing Nitro Oxide are being re-routed back to the cylinders to be burned off again. But at the same time, oil mist from the crankcase also enters the manifold. The combination of oil mist + exhaust from EGR = soot / carbon deposits, which you saw when you open your manifold (for others refer to photo ref #1 below).
So doesn't matter Diesel vs Gasoline, if you have EGR, you'll soon see soot 😁but because Diesel produces Nitro Oxide more than gasoline, you'll see EGR being enforced for Diesels in environmentally-friendly countries.
Solutions to the Soot:
1. Yield to the Manufacturer. This is when everyone says "if Mitsubishi engineers are so smart, why they let this happen?". So EGR is a Law-thingy, and they cannot sell in Europe, America if they don't comply. If you accept this solution, then we can just chin-up and pay when the lazy mechanics say "oh, this soot is too much, we cannot clean it, let's buy a new intake manifold" or "the soot came back within months, really? Then we need to replace your EGR Valve, it may be malfunctioning!"
2. Delete EGR Valve + Blanking Plate: this removes the valve that close/open to allow gases to re-enter the manifold and block the hole for EGR with a plate. Note that modern engines open/close valve electronically and have sensors, so assume the Computer does some assumptions and calculations based on EGR. So if you delete EGR, it may throw Check Engine Light. Is it safe? EGR is turned off during idle, otherwise it will cause rough idle, so technically with EGR off, engines can still run fine. Deleting EGR is like Deleting Catalytic Converter, it will throw CEL, it's bad for the environment, but car will still run fine.
3. Prevent Oil Mist from re-entering to meet with EGR: this is Catch-Can. But if you do #2, which is more effective, you don't have to do this Catch-Can thingy. If you love the environment and don't mind emptying Catch-Can every month, then this is the solution you'll choose.... until you find out that your Catch Can is still producing soot every few months 😂
Good luck with your choice, and update us after few months of implementing it 🙂