I'm no expert, but as I understand it , switching from fully synthetic oils to semi synthetic ( hydrocrack ) or mineral oils presents no problem.
What you are proposing however does.
One of the properties of synthetic oils is that they act as a detergent and tend to dissolve the carbon build up on piston rings etc. thereby actually reducing compression. In addition the oil seals on your engine will now be hardened after prolonged use , which will lead to leaks if you use synthetic oil.
Synthetic oils are superb lubricants if they are used from when an engine is new.
I run a Volvo D3 110 and use Shell Rimula 15/40 , a heavy duty marine diesel oil which has the same specifications as Volvos own brand , but half the price.
Hope this is of help to you.
PS Moderators. Can you delete the entry ref. Shell Rimula if I have contravened Forum rules.