5W30 or 10W30 synthetic oil is fine. There are companies that make light weight synthetic oils for maunal boxes but I've not seen that those were easily available. If you have an M46+OD you remove the bottom plate and let OD drain also. The remaining ATF is not going to hurt anything. Years ago there was a fix for neutral, clutch released gear rattle where you drained atf and used regular 10W30 motor oil. It fixed gear rattle and made trans shift better when hot ( this was in SE USA). The OD had better more positive engagement as well. I had a 240 w/M47 that was very hard to get into 1st if you put it in neutral and then pushed in clutch and attempted to go into 1st. Use of synthetic 10W30 made it shift like a knife through warm butter. The manual for these gearboxes has a note on the page for specifications that references an asterisk from the oil type in the main text that says to use Volvo thermal oil part number xxxxxxxxxx if there are complaints. And it says not to use it if temp is consistently below -10 C (14F). The part number listed is not carried in the US.