I too have read a lot of various things on the Net regarding our Spanish Ladies.
I have no problem using both surplus 7.62X51 and commercial .308Win in my FR8 rifle. I do NOT subscribe to the old "soft Spanish steel" canard. I DO check my bolt lugs for setback often.
My old M43 Mauser in 8X57 is dated in the 50's, as is the action of my FR8. The M43 I have been shooting since I bought it in 1976 - never seen any problems.
My FR8 and M191 are more recent acquisitions - I check them often too. I load 7.62X51 for both - full power GI spec for the FR8 and CETME, and lower-pressure loads for the old M1916 - just to be safe. I keep the lighter loads marked specifically for the M1916.
On another forum, a member in Spain with Spanish Army experience provided a translation of the Spanish manual for the CETME - it indicated they used 7.62X51, as made in Santa Barbara, Spain. This is NOT 'NATO' spec ammo, but then neither is the South African or the Australian ammo I have stocked and shot. All perform very similarly, and are of similar accuracy. I see little difference in POI between them - so their loading standards appear to be similar.
I think much confusion comes from Century and other importers stamping these rifles as '.308' when they are NOT.
My Spanish Mausers......
(The M43 Airforce Model in the middle currently wears a shortened Brazilian '08 stock)