BMW M30 engine; durable if properly maintained.
Despite being the functional equivalent of the small-block Chevy engine for BMW, the M30 has a few idiosyncrasies with which buyers should be familiar. There's a weak spot or two, but neglect is usually required to bring them out.
Early (1977-80) M30s suffered from a horrible EGR system that created excessive heat and warped cylinder heads. There was a class-action lawsuit, and BMW replaced most of the heads with an updated design that did not warp. The better solution was to remove the offending EGR system and replace the U.S. smog exhaust manifolds with free-flowing European exhaust manifolds. However, when the government of California lost its mind years later, and other states followed, sometimes the EGR system had to be refitted for smog testing.
Up to 1985, the M30 was a bit underpowered, due again to U.S. emission controls and low compression. Things got better in 1985, and from then on a Conforti chip really awakens this engine.
In 1989, the E34 5 Series finally received the 208-bhp European version of the M30 engine.
The M30 engine is very durable if well maintained. The problem is, most aren't. Valves must be adjusted approximately every 15,000 to 20,000 miles, and at the same time cam oiler bar torque should be checked unless the cam oiler bar banjo bolts were replaced with the new thread-locking style (pn 11 42 1 738 621, 2 required, 4 pn 11 42 1 252 343 washers also required). Coolant needs to be changed at something approximating a two-year interval, and phosphate-free original BMW antifreeze is highly recommended. Given these procedures, high-quality motor oil in the proper viscosity (20W-50 summer, 10W-40 winter, generally speaking) and changed at intervals appropriate to the product, the M30 can last hundreds of thousands of miles without the need for a teardown.
Failure to adjust valves and a loose cam oiler bar results in a worn camshaft. Failure to change coolant and use the right stuff results in aluminum oxidation formation inside the cylinder-head coolant passages, creating a hot running condition and eventual head-gasket failure. In this event, you have to do a complete valve job, disassembling the head so it can be bead blasted inside--this is the only way to get rid of the aluminum oxidation. If you just replace the head gasket, it will all happen again.