There is only one reason I see to use RG8 over 8X at HF frequencies, power handling capability. 8X is rated at 1kw at 10 MHz and 370w at 50 MHz. From Belden's website.
If running full legal limit use RG8 or equivalent, if less RG8X will work.
No matter what you use buy quality coax from a dealer you trust.
Strange because no one else rates 8x that way. Its rated 1500 watts at 10mhz and 800 at 50mhz. Even 58 is rated 425 watts at 50 MHz
Yep, no one else....except the companies that manufacture the cable.
Belden has their ratings. So does Time Microwave, and they provide an outstanding calculator to figure it all out:
At 10MHz, 85 feet of RG-8X is rated at an average power of 620 watts only. At 50.125 MHz, that drops to an average of 270 watts. Note, these are averages, not max power. What happens if you exceed the ratings? Well, at the least your cable will get very warm which can cause cable breakdown; at worst it could fail rather dramatically.
Want to check out the figures for yourself a variety of cable types (Times Microwave and non-TM), Times Microwave provides an EXCELLENT calculator. The info provided includes things like calculated cable loss, assembly loss (cable + connectors), efficiency percentage, and other factors. Here's the link to the measurement for 50.125 MHz. You can change the parameters (cable type, length or frequency) and it will give you the figures you need/want. http://timesmicrowave.com/calculator/?productId=121&frequency=50.125&runLength=85&mode=calculate#form