Any advice on the oil?

The obvious first question is what are these signs that the oil is "getting too thin"? How are you measuring the viscocity? What are the symptoms? What is the problem you are trying to fix?
In my experience you can run these engines very hard on the bare minimum of servicing using the cheapest oils and they will easily outlast the rest of the car. I've been on this forum and a few Jimny facebook groups for a decade and I can't recall any instances of lubrication related failures (e.g. crank, rods, pistons, cams etc.).

wrote:

also what does FSM mean?

Factory Service Manual

oh right i should have guessed that

my lifters are loud, and i noticed that the sound goes away completely when i add fresh oil and it gets worse when i push hard and (i think) the oil gets really hot, and in general i want to avoid any risks of improper lubrication under really heavy loads, like when i keep the revs over 4k when practicing rallying offroad.
also, would you guys happen to know any good ways to ensure proper oil pressure when the car is going through lateral g loads? i heard that long curves at high lateral Gs can starve the oil pump

Firstly I agree with what has already been said about using the grade of oil specified in the owners manual, this will probably be 5w30. Second I work my Jimny hard, not necessarily at sustained high revs but regularly at high loading both carrying and towing. I had over a hundred thousand miles on my last one with no ill effects and fully expect the same from this current one. Third, m13a engines are cheap enough and easy enough to swap if you do somehow manage to kill one, which is unlikely. Fourth, attempting to second guess the research and development department of Suzuki with regards to something as important as the lubricants of the engine is I think something of a fools errand, they spent much time and money on getting it right.

Just out of interest how old, how many miles are on your Jimny? The reason being that a healthy engine should only offer the light tick tick of the injectors as they fire, your comments about the lifters being quietened with new oil makes me think this engine is starting to wear and for what you are wanting to do with it it might either benefit from a rebuild or want replacing with a newer and or bigger one. Also you mention getting a diesel bonnet for the scoop, this will not help with engine bay temperature, you need to give the hot air somewhere to go either by fitting vents or spacing up the back edge of the bonnet, trying to force more air in the front isn't the answer as you will be fighting pressure under the bonnet.