Good to know, when I rebuild my 4.0 it'll be stock stroke but I may turbocharge it (with forged pistons and beefed rods). I did always feel like the radiator on my XJ Cherokee was a bit small for an all-iron 4.0L straight-6 designed in the 1960s. I overheated it many times while offroading until I installed an aux electric cooling fan; apparently since mine was a manual trans with no A/C it only got the engine-driven cooling fan from the factory which only pulls on half the radiator core. Might have some design flaws but the thing is so damn bulletproof it can take being overheated and run with dog-piss in the oil sump lol. Mine started having weird oil pressure issues likely due to me revving it to 4500 RPM on a regular basis, pretty sure the con rod big ends got ovaled and the bearings are bad. But that Jeep got totaled from being rear-ended a few months ago so the engine is now in storage.
On the subject of the cooling system, the heater also sucked lol. Might be because with no A/C it didn't have an air-recirc function so it was always pulling in outside air but that thing wouldn't put out heat worth a damn basically until the thermostat was opening. And I did a lot of maintenance and small mods to the cooling system to increase flow (took out heater control valve and replaced with higher-flowing brass ball valve, added Water Wetter to coolant). Got it working better but still sucked compared to my other vehicles.
Back on the subject of oil, I am curious about doing an oil analysis on my vehicles. Would be nice to confirm with actual data that I can extend my oil change intervals past 3k miles. And same as @YY1 I run Valvoline full synthetic in all my engines (Duster has a roller cam so it gets the 5W-40 Euro-spec MST stuff), good price and IMO better than Mobil 1. Supposedly Pennzoil is a lot better than it used to be but I don't feel a need to switch when the Valvoline FS has been serving me well for over a decade.
