EGT is more relevant in Diesel engines, particularly turbo diesels than it is in petrol.
I wouldn’t rely too much on EGT gauge for AFR. If you want accurate information it’s better to fit a wideband AFR sensor and gauge.
RE the sludge, I would put a little bit of paraffin in the engine oil, run it for 10 minutes and drain it while it’s still hot. Repeat as necessary. You can buy bottles of engine flush but it’s basically paraffin with a fancy label.
that's where the exhaust temperature gauge comes in handy to really tell you exactly where things are going when you are throttle happy....the cooling system is fine even i hope the oil quality is good as he has been here for few oil changes and i dont see him trying to put camel oil to save a few bucks...its that the exhaust gas temperature gives you an instantaneous picture of exactly what is going on inside your engine. As loads build, exhaust gas temperatures rise. If they’re rising too high it’s a pretty safe bet your engine is running hotter than a pressure cooked fryer … and will no doubt culminate in a fried head
this also serves as to see a side of your engine that is honestly far more important than what your coolant is doing. It can tell you if your fuel mix is too rich or lean, your exhaust is too restrictive, and if you need to ease off the right foot as you’re climbing a hill.