Cams are def. the B18/20 weak spot. The lobes aren't really well oiled, nothing direct anyhow, I guess they get some of the oil draining back down from the rocker shaft, and crankcase windage. But that's about it.
That and apparently there was a long period where replacement Volvo lifters were not quite up to snuff.
That and apparently there was a long period where replacement Volvo lifters were not quite up to snuff.