[Reece]

Best to use like for like. Yellows and orange dips and the like to replace old paper/wax caps, and micas for micas as they are in more critical circuits. Radios from the sixties you're probably OK leaving the micas and ceramics alone, but older than that, some may be starting to be questionable. What happens is sometimes the seal fails where the wire enters the outer casing, or the casing splits (especially when you try to turn one over in the circuit to read the value) and moisture gets in. And some marked Micamold are actually paper caps: they'll often have their mfd. rating in figures rather than in dots. Their position in the circuit and value would tell if they should be replaced with poly or with actual mica: mica in RF duty, poly in audio and bypass locations. Personally I leave micas and ceramics alone unless there is a problem. If you do a wholesale replacement in an RF circuit you may get the alignment out of whack, not that you wouldn't tweak it in the end anyway.