If it had oil changes every 3 months I wouldn’t be worried about sludge.
It’s oil that hasn’t been changed often enough and forms into puddles pools of gunk inside the engine, rather than nice clean free flowing oil that should be there doing its job. This gunk can block off the oil paths to and from the cylinder head, and if that happens the lack of proper lube takes its toll first on the variable valve timing mechanism. Something you don’t want if you value the contents of your wallet. That mechanism is very sensitive to oil sludge’s effects. Provided the manufacturer’s recommended oil & filter has always been replaced according to the time and mileage recommendations, not likely to be a problem. If it exists, oil sludge is usually most easily visible under the valve covers. Sometimes you can peak into that hole where you pour the oil with a flashlight and see it without removing the valve covers.