I would avoid using petroleum jelly (Vaseline) as it does degrade the rubber over time.
If you want a safe, easy to find lubricant (available at any drugstore) try K-Y jelly. It is safe for rubber such as latex and helps assist you in inserting the male part into the female part easier.
Oh, and it works for subframe bushings and other car parts too 
But seriously, KY Jelly works just fine for rubber parts and is completely safe.
Just don't buy the KY along with 5 boxes of latex gloves (the ones you buy to keep dirt off your hands when working on the car). You get weird looks at the drugstore. Ask my how I know
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