Hydraulic fluid question

[Bruce Parker (RIP)]

I've installed hydraulic clutch and accerlator systems in my old girl and need to bleed them. My question is regarding which fluid to use, regular or synthetic. I hear synthentic is better from a corrosion point of view especially if the cylinders won't be getting regular use...BUT...what if some of the rubber cups may have seen regular fluid some time previously? Some cylinders are new; some look new but 'smell' like regular fluid has been in them (as a lubricant on assembly perhaps). The guy who made my hoses says you can use synthetic in systems that have been filled with regular, but once you go synthetic, there's no going back. Can anyone confim this is true? Should I stick with regular fluid? The clutch master is in a particulary difficult place and I really really don't want to bugger it up.