The check valve update is the only significant change I see in the rule book bulletin today with wording adding to state that it is for superspeedway events until the end of the year. This is likely in response to the the valves sticking and oil tanks bursting at some tracks.
More about some of the valves sticking and oil tanks bursting in this thread.
The first incident related to that sticking valve that I am aware of back at Pocono.
Then this one at the Indy road course is the next one that I heard which had a sticking valve.
The last one I heard of came at Road America.
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