Doing a compression test while it is running well is a great idea. Just hope you never need to use the data later.

I am a big proponent of PM. When I got my cars a few years ago I went through them and replaced a lot of things that still had some life left in them.
I see nothing wrong with it if you are in these cars for the long haul. I figured they were about half used up, so I went ahead and rebuilt the front ends, rear subframe bushings/diff mount, flex discs, center support bearing, brake hoses, shocks. My rationale is that , once done, these should be good for the lifetime of the car (or mine), and will restore the car close to the way they left the factory, performance-wise. And that also took some of the risk out of breakdowns etc later on. Of course, I still have old engines and transmissions to worry about. I am slowly rebuilding a 300D transmission from 2 parts trannys I have and one probably good one. I know that sooner or later the one in the 300D will fail and I will have a rebuilt one to slap in- or I may just take the opportunity to swap in the fresh one before the tired one fails.

Rick

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)