Common Format; Kindle Oasis and Fire HD 8

Hi all,

I have a latest generation Kindle Fire HD8 and Kindle Oasis.

In the past, I have had trouble getting the Fire HD8 to read mobi files created with Calibre. Those files worked fine with older regular Kindles.

Besides downloading other software (which is my current workaround); is there a format that works on both the Oasis and the Fire HD 8? I would prefer to use the standard Kindle reader on the Fire.

Thanks.