I use an Ebonite Gyro from 1991 that is used for 4/7 and 7 pin conversions(lefty). When the lanes are extremely dry,the Gyro does pretty good without burning up.

That is the problem with today's weak balls .. There is nothing that is a truly weak cover.  They all jump pretty hard and all can cover 15 or more boards off of any decent amount of friction.  Maybe I'll look for an older truly weak cover ball.

I have thrown both the original Natural, and the new Blue Hammer in the manner you suggest.  My spare shooting with both is better than it was with polyester.  I have also had good success with both as condition specific strike balls. 

The key is to keep the cover tame, like 4000.  Also remember that I said condition specific.  If you just get the urethane out anytime the lanes seem dry you will be disappointed.  Urethane is most useful when you can play relatively straight from outside 10 board.  If you are forced to play much out angle you will be disappointed in the carry no matter how many revs you get.  I actually think urethane is better for us straighter low rev strokers than for power guys

Part of that is the lane surface being plastic...it creates more friction than wood down lane thus the balls continue to potentially hook, as well as the pin decks being phenalic minimizes skid out...
This is the issue with urethane, no matter the surface...they create an earlier response and confuse bowlers..they react sooner and less down lane which confuses bowlers that are used to seeing motion down lane...it's similar to compounding the lane
Now granted, this also depends on where the friction is, if it is wet dry front to back, then urethane can overcome it but hook in the front on away from the pocket (side to side) decreases potential of the urethane
You can take a weak core/cover reactive and use a flare decreasing weight hole to minimize overall reaction also

Great info. I was just thinking urethane since I could use it for spares and also get a different look for those times when I might need it. I am expecting and also like the look that urethane provides.   I will also probably carry another medium -light oil ball as well.  Looking at a Hy-road pearl.. Not sure that is really that weak though.. I also have an old game breaker that I drilled to hardly flare and be smooth.  That ball seems to work really well when I don't want to cover a ton of boards.

I have both the XXXL and pink hammer and have them both at the same surface and polish (1000-2000-4000+black magic XL).  The XXXL has more length but more backend reaction than the pink hammer.  So if the heads are hooking early, the XXXL is a better choice as I can swing it and the core will allow it to recover and carry.  While the pink hammer is more useful for short patterns when the heads start to go if I am using them as a strike ball.

And the pink hammer is the better spare ball because of the weaker core.  However, on sport patterns, the pink hammer will not bite at all and the XXXL will grab a board or 2 if I need it.

Also to consider, Motive has the artic sniper which is urethane and the weakest core I have ever seen.  One guy had one in travel league last month and he said it is the weakest ball he has even thrown.  I may look into it.

The biggest difference between polyester, urethane and resin is how they respond to friction...urethane being the earliest or slowest...no matter the condition bowlers still look for motion down lane...it's easier to visualise and adjust to