Dumping additive in the fuel side isn't going to help the oil side is what has trouble.
The glowplugs can be tested to see if they work properly through the UVCH (under valve cover harness) the outside 2 pins on both sides of plug are for glow plugs. Take a voltmeter ohm between battery ground and each of those pins. Spec is 0-6.0 ohms. Anymore than that they are no good
Buts likely the injectors.
You mean 0.1-0.6 ohms.