Motor oil works. Also if you're tapping aluminium I would recommend kero or metho. They work well on aluminium. Or you can buy trefolex, rocol, tap magic, Neat oil or even use straight soluble cutting fluid without diluting it.. all of these will work on steel or ally... whatever you do, don't make a habit of using them dry, it will wear them out in no time
BACON FAT!! That's OLD SCHOOL AS!
Still works well though,,, although kind of gross hhaaha
If all you are doing is cleaning old threads, your carbon steel taps will be fine, but listen to them
if they start saying "STOP" then stop, or back off 1/2 a turn and try again gently. They're more brittle than the HSS taps so they're less forgiving if you're heavy handed.
Like 1018 said, if you break any or wear any out replace them with good ones, but there is nothing wrong with what you have if you're only using them once a year. I would like to add FEW and Dormer to that list, lately I've found that the Suttons ones are going through a weird phase of being a bit brittle.
You'll know they're worn when they start getting hard to use. Stop using them when they get to this stage because it is easy to force them and break them.
If you're tapping new holes, make sure you drill the right size hole. If its too tight it will bottom out the root of the thread on the tap and load it up making it way more likely to snap. There are hole tapping charts everywhere, I can't remember the drill sizes for the imperial stuff because I rarely use it at work, but the rule for working out metric drill size is:
Major diameter - Thread Pitch = Drill size.
So if for example it is an M6, the pitch is 1mm.... so its an easy one, Its just:
M6 - 1 = 5mm drill...
M8 is 1.25mm pitch so its M8-1.25=6.75 (just use a 6.8 )
The formula for working out imperial threads is a bit of a turd which is why I never committed it to memory
