I have the same car, it has mechanical lifters. I would bet Firestone got dirt or a piece of something into the oil fill hole and it plugged a gallery. Then it broke loose. That model Purolator made filter has an excellent adbv, cut a number open to see myself. That wouldn't matter for flow anyway. If the louvers are closed there is still bypass. If the oil pressure is so low the lifters aren't getting oil the main bearings would be under a lot of stress as well I would think. The last time I had an oil change at Firestone, years ago, the dipstick was left on top of the engine. I guess it could be a chance mistake.