Filling them like that would make them last virtually forever I'd think. Do
you think it transmits enough extra vibration to be felt at any significant
level?
I have a Jaguar XJS V12 that I replaced all the rubber bushings with urethane. Trans, Motor, steering, shocks etc. Luckily the aftermarket
had all those for sale at a reasonable price.
The GOOD. It handles so much better and more precisely due to the rack
being now mounted in urethane. At least now they will last. Almost
all old bushings had turned to GOO. No kidding, goo.
The BAD. Urethane on the Jag does increase noise and vibration to a very small extent. The V12 is so quiet that you can hear any extra noise. The increase isn't much, but I could hear and feel it at first, but after that first hour I no longer notice it. I doubt anyone would notice unless they had a fast before and after as I did.

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85 300TD FED-Daily
84 300SD-Wife's
86 XJS-Sunday
66 GMC-Work- Given to my stepson
83 BMW Airhead- Given to my stepson