Opinions vary on exactly when, but it is a bad idea to change to synthetic in an "old" engine. The reason is that the synth flushes out certain additives which have "swelled" the seals, which then shrink and start to leak.
When you ask "How old is old?" it gets a bit vague, but maybe after about 80,000 miles the seals have aged too much to support the change.
I did it on a Mazda V6 (so very similar to a GV V6 engine) which had done about 59,000 miles and it was fine - the engine hardly used oil at all, and didn't need topping up between annual changes, right up to 98,000 miles, when I sold it.
I've since done it on three Suzuki engines (all below 50,000 miles) and they hardly use oil.