The naturally-aspirated V8s are held up as the definitive reason to buy a W463: the M113 (and later M273) are called the most reliable gas engines Mercedes ever made, with 500k-plus and one documented million-mile G500. The M104 inline-six and the OM603/OM606 diesels earn the same 'million-mile machine' reputation, making high odometer readings far less scary than on other Benzes of the era.
Engine longevity: M113/M273 V8, M104 I6 and the diesels
Mercedes-Benz G-Class (W463 1990-2018) · mechanical