Hi grizzy

why you want to move for Semi-synthetic oil?

Semi-synthetic is expensive than mineral oil , also in old and large system you have a lot of in system, to to drain them it is difficult or we could say almost imposable. some compressor manufacture does not recommend to do that and some of them does not answer you .

also in large system we change oil compressor every 1000 h in (R 717) NH3

But if you want to do that I recommend first you test it. HOW?
you have mixed old oil with NH3 (from oil drain ) mixed it with your new oil and put it in different temperature +50( condenser temperature) and then near pour point (( I think pour point for Clavus 68 is about -30 )) so -25 for for 2 or 3 weeks and then check the mixed oil, how is that ? special acidity of oil and those it comes greasy or it has good condition . then go further.

changing type of oil in semi hermetic system ( close system ) is risky with out getting permission form compressor manufacture.