I have a 304 Jinma, very similar to your 354 just a couple fewer horsepower. I do much of my bush hogging on hillsides and generally use L1 or L2 and use L3 or L4 on the flats. Even H1 is a bit too much ground speed for clean mowing with my cutter. As advised, I strive to keep my throttle set so the engine is operating at peak torque, about 2000 rpm on my 304. For plowing, you'll almost certainly not want to use anything higher than L2 or you risk damage if hang the plow on something and can't get shut down quickly enough. Always bear in mind that regardless of how well you're going forward, you will only ever have two brakes on the rear wheels for stopping.
I'll also recommend that if you have a loader on your tractor, try to avoid using 4WD when using the loader. You risk damage tot he front drive train if the rear wheels spin when in 4WD and the front is loaded with a heavy bucket. Of course, trying to use a loader like a bulldozer will almost certainly break something. For situations where you're in 2WD and the rear wheel slips, use the differential lock as the first resort - you'll be surprised at how effective it can be. It is self-releasing so no worries about damaging something after you get moving again.
I'm in the Virgin Islands, a mostly tropical zone with temps between 64F and 94F year-round and I use 85W90 gear oil in the transmission and axles and differentials, 15W40 motor oil and AW46 hydraulic fluid for the hydraulics and steering pump. I have found the 85W90 gear oil to be just right for the gear box. I initially tried AW32 hydraulic fluid in the lift box but found that it was not as good as the heavier 46 viscosity fluid. I've never had any reason to try anything other than the 15W40 motor oil.
Note - the injector pump needs to be kept filled with the appropriate amount of motor oil. It should be changed when you do a 6-month service.
Use a brand name SCA-type coolant designed for wet-sleeved diesel engines and keep it fresh. Keep a close eye on the radiator fins for blockage from chaff and dust and clean them regularly or you'll experience overheating. The air filter canister blocks a good bit of radiator flow so it operates at the edge of marginal. The thermostat is an 85C stat and some don't seal properly so the engine doesn't come up to heat quickly enough. The radiator cap is only about a 7psi rating - don't mistakenly replace it with a higher pressure cap or you can damage the radiator.
There's a bit of information that I've gleaned after a couple three years of owning my Jinma. As noted, the manuals are not that well translated from the Chinese and can be confusing. There are a lot of guys here with the knowledge you will need so just ask about anything that is unclear.
Enjoy your new tractor! These are amazing little workhorses.
