I guess it all depends how far you want to go with it. One can under maintain, and also over maintain things. Ill try to tackle your questions in order from start to finish
So i personally maintain my rigs when i see a sign of something failing or something which could lead to failure. For example, i found a few bent bolts yesterday so ill be removing those and replacing them tomorrow. That said though if im planning a big trip or trail walk ill give the truck a good check over the day before to make sure i dont get caught out miles from home. With trail rigs, it tends to be things going rusty or needing lubrication which leads me onto your next question; what to use for that, and when. Again all i can do is give my opinions, so you might feel differently I use 3 in 1 oil on metal cvs and uj's, but avoid lubricating plastic sliding ones as dirt sticks to the lube and this can keep them from operating correctly. Instead, just keep them clean and free from dirt after runs. In wet conditions you can use grease on these parts (still not the plastic cvs IMO), instead of 3 in 1 to get a more water resistant coating; which i would do if i were running in streams and things like that. In dusty summer conditions you can use a dry lubricant such as silicone spray to avoid the dust sticking to the lube and creating that gross grinding paste which eventually ruins joints and cvs. For gears and gearboxes, id follow the manufacturers advice which should be found in the instruction manual for the venture.
Now, thread locking compounds. Thread locker, or loctite should be used in situations where a bolt is threaded into metal and subjected to vibrations or other stresses that can cause bolts to gradually work loose over time. Again, the manual should tell you where to use this, but as a rule id use it anywhere which is under stress or vibration and the bolt threads into metal. There are many different types of loctite, generally youll find green, red and blue. From my experience, the blue stuff ive got at the moment is way too tough for rc applications. Green is supposed to be perminant, red is very strong and blue is supposed to be mild. My friend uses this stuff called loctite 222 which acts as a thread locker but still allows you to undo the bolt when required; this is a light pinkish colour. Bare in mind anything you loctite has the potential to strip if too much is used, so sometimes its better to leave well alone with the compounds. A little tiny dab on one side of the thread on the end of the bolt is usually enough to stop them working loose. If they still work loose, use a little more Remember to pick dirt out of the heads of screws and bolts before attacking them, as this aids your ability to get torque on the fastener without slipping and damaging the head; which is a huge PITA. Good quality tools are necessary when working with these tiny bolts I personally wont ever use threadlock into a motor, or an engine, or anything really expensive just on the off chance that the compound sets like hulk snot and wont allow you to get the bolt out again!
Shocks need refilling when you can audibly hear a kind of "squelching" sound from them. This is the air in the shock mixing with the oil as its forced through the holes in the pistons inside during the stroke. To fix this, simply top up with the same weight or CST oil as its called now and youre good to go The shocks on my ascender leak badly, the truck will often tilt to one side as rest which shows me which shocks need to be topped up. The oil also needs to be changed if its contaminated with dirt as this will destroy the inner pistons ability to seal. Another trick is to look for the characteristic slippery patches of oil on the truck around the suspension. Luckily this will collect dirt and dust like a magnet leading you right to the cause! this is the same with gearbox leakage ect.
The manufacturers will always point you toward their "proprietary" blended oils but usually these can be substituted for other bulk buy types of the same item. For example i bet HPI sell a grease specifically for the gearbox on that venture, which is really expensive. When actually you could use standard lithium grease which is much cheaper and will do the same job. From my experience, CA glue or superglue is usually all the same, but stuff for tyres has been made in a low viscosity formula for ease of use. Oil is just oil IMO, as above 3 in 1 tackles everything on my ascender metal to metal and ball ends, while silicone spray is used on plastic to plastic driveshafts.
Its all quite basic logic, if in doubt have a little read about uses and properties of grease before you use it, as using the wrong one can in fact ruin parts. Luckily these little trucks are pretty resilient and as you say, youve already used it for many hours without issue or any mainaintance at all Ive done hardly anything to my ascender in that department and ive owned it over a year now. If you need any advice or suggestions feel free to PM me id be happy to help
Cheers, NB.