13 SR5 down Shifting Normal at 60000

I switched my oil to synthectic oil at 60000 miles and now when I accelerate it make a groaning sound, also when slowing down,it feels like the trans is down shifting when stopping. I went to Toyota dealer to have the tranmission changed and still same issue, the issue go away when I have been driving for a good hour or so. Is this normal or is there something I should check. The oil change may have nothing to do with but strange

 

The transmission is supposed to downshift when stopping...

 

It's strange that I never notice it until now,when I have been driving for awhile its stop downshifting when stopping its that also normal.

 

Just because you don't feel it after it is warmed up doesn't mean that it isn't doing it.

Check the fluid level.