Hi there
If I understand your question correctly, you were asking if the external oscillator being used is supported by FPGA. We don't have specific guideline for external oscillator. You need to adhere to clock/pll requirement stated in datasheet below:
https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/cyclone-iv/cyiv-53001.pdf
Referring to 2 KDBs below, Intel doesn't provide following clock input spec:
https://www.intel.com/content/www/us/en/support/programmable/articles/000083116.html
https://www.intel.com/content/www/us/en/support/programmable/articles/000084779.html
Thanks.
Eng Wei