912ULS Oil Pressure Relief Valve

Those are the correct part numbers.
The shim is only needed if the oil pressure is too low. (< ~35psi).

The Mushroom Head Valve Piston is meant to replace the Ball Valve if you are experiencing Oil pressure instability.
The solution does not always help.

If you do not have an Oil pressure problem, changing to the Mushroom Style Valve will not change anything other than lightening your wallet and maybe give you a sense of accomplishment.
The "Newer" design is not necessarily the "Better" design.

If it "Ain't " Broke, Don't fix it!

Yes, but stay away from the shim. Most owners cause more issues with shims because they never double check the REAL oil pressure with a mechanical gauge in conjunction with their regular instrument gauge. Before you EVER use a shim you need to confirm a real low oil pressure which is very rare. I said REAL low oil pressure, not one that the instruments says. It's wrong more times than not because of the pressure sending unit. The metal mushroom that replaces the ball bearing many times will raise the oil pressure around 5 psi. I've probably installed 70 plus of the mushroom's over the years. I used to buy 10-15 at a time when they came out. 

Thank you for replies...

What I’m concerned about is the amount of time it takes for oil pressure to develop once engine is started.  I pre-heat in cold temps, but even when OAT is 70F it takes better part of 15-20 seconds to start building oil pressure.  Running pressure is 45-50 psig on Dynon D180.  Engine has 850TT and I’m using Mobil 1 Racing 4T 10W-40 Full Synthetic Oil since new.  Thank you for replies...

If it's slow to come up it may not be your Rotax engine, but the way things are plumbed and or the sender. The sender is a vary common culprit. Your plumbing may need cleaning or an inspection to make sure it's not part of the problem.

When you initially start the engine what are you idling at for your 20 seconds? 

Roger - thank you for your reply.  Your knowledge of Rotax is invaluable.   

I idle 2600 - 2800 RPM to achieve nice smooth running at startup.  I prefer to not have gearbox chatter with engine running slower speed as it warms.  Once warmed, engine idle is smooth and I stay ~ 2200 for ground ops.