90hp 4 stroke suzuki outboard how do i reset the oil alarm?

Agree, fuel delivery system. Probably needs replacing.

being 1984 iot is possible that it is a 2 stroke
simple check

does it have removable rocker covers

Yes,

The basic circuitry is identical for identical connectors.
Additional harnesses are avalaible for newer dashboards etc...

first bit of advice
do not use e-mix fuels in outboard engines
WHY
because ethanol separates from the fuel if not used continuously so letting it stand between outings is not good
secondly ethanol is hygroscopic which means that it absorbs moisture from the air that is going in via the tank vent and that results in corroded aluminium parts and rusted fuel tanks and outboards operate in a very moisture laden environment
now for the problem
being a late model engine , it should be a 4 stroke and like any 4 stroke engine , check the timing belt/chain, crank/cam position sensors , ignition module
it is important to check all harness connections as salt water rapidly causes corrosion in electrical connections and components
have the service agent do a scan for fault codes remember this
Suzuki out board 2 stroke motors were used in the early suzuki 4x4 cars and modern 4 stroke engines are simply car engines on end

Frans, Check the attached links,instruction and guides, Good luck
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info."
suzuki 100 hp outboard specifications Google Search
Suzuki
https://www.tradebit.com/filedetail.php/142965517-suzuki-outboard-service-manual-df90-100-115

Matt, If engine is two stroke and not oil injected then yes you must mix 2 stroke oil and fuel. 4 stroke engine have a crank case which is filled to proper level with 4 stroke engine oil. Check the attached links,instruction and guides, Good luck
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info."
How to Decide Between Two or Four Stroke Outboard Boat Engine

4 Stroke Outboard Motor vs 2 Stroke Outboards

2 stroke outboard Vrs 4 stroke outboard

http://www.cottagetips.com/tips/boat-motor-gas-to-oil-ratios

go google and type in--- workshop manual for suzuki 140 hp 4 stroke outboard and there are manuals that are available on different site
the manuals will show where the lines are
one site --servicemanualspro.info--- has a pdf down load link

I have a 25 hp merc outboard that starts right up but idles high then low and when running it just dies . Is it a choke problem?

might be some electrical problem, because, Suzukis are mostly controlled Electronically, it may represent a malfunction!!
try contacting an electrition!!