What sort of oil for my v8oo suzuki gearbox ,is it easy to add oil in the fork where can j get a rack and a top box for my helmet,thanks

The amount of oil that goes in the forks of a 1982 Suzuki GS 750T will depend on the type of forks that are installed on the motorcycle. It is important to use the correct amount of oil to ensure that the forks are functioning properly.
If your Suzuki GS 750T has standard telescopic forks, you will need to add 10.8 ounces (320 mL) of fork oil to each fork. If your motorcycle has air-assisted forks, you will need to consult the owner's manual to determine the correct amount of oil to use.
To add oil to the forks, you will need to remove the fork caps and use a fork oil level gauge or a piece of wire to measure the amount of oil in the forks. If the oil level is too low, add oil until the forks are filled to the correct level. If the oil level is too high, remove excess oil until the forks are at the correct level.
I hope this information helps! Let me know if you have any other questions.

Question edited for clarity.

RM85 Fork Specs. Bound to be the same forks

When adjusting the fork oil level, compress the outer tube fully
without the fork spring.
? 09943-74111: Front fork oil level gauge
? Fork oil level (Standard): 124 mm (4.88 in)
? 99000-99001-SS5:SUZUKI FORK OIL SS-05
? Standard fork oil capacity (each leg): 351 ml
or an equivalent fork oil
(11.86/12.36 US/lmp oz

Free Service manual for the RM85

https://www.manualslib.com/manual/824675/Suzuki-Rm85.html?page=164&term=fork+oil&selected=1#manual

Lots of Service Manuals for around $34

https://www.ebay.com/itm/385931112664?hash=item59db4970d8:g:WyUAAOSwkFpj2-n~

More here.

https://www.google.com/search?q=1986+suzuki+rm80+service+manual

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Take them to someone who specialises on suspension rebuilds and get them sorted properly. You cannot afford to mess about with the handling on a bike that is capable of 200mph!

The gearbox IS part of the block and the oil is shared between the gearbox and crankcase, so yes, this is motor oil. That little plastic cap is the top of your oil pressure switch/sending unit and there is supposed to be a wire attached to it. This part is relatively inexpensive from the dealer and quite easy to replace.

If the gearbox oil is REALLY dirty it can affect shifting.The only other thing will be a shift fork going bad. Replacing a shift fork is not generally a do-it-yourself project. The dealer will charge $750 or more. Sure do hope you have dirty oil...

FORK OIL
• Place the front fork vertically without spring.
• Compress it fully.
• Pour specified front fork oil up to the top level of the outer
tube.
 Capacity (each leg): 490 ml (16.6/17.3 US/lmp oz)
 99000-99044-L01: SUZUKI FORK OIL L01
or an equivalent fork oil
• Move the inner rod slowly with the special tool ten times and
more until no more bubbles come out from the oil.
 09940-52841: Inner rod holder
NOTE:
Refill front fork oil up to the top of the outer tube so that bubbles
are visible while bleeding air.
• Refill specified front fork oil up to the top level of the outer
tube again. Move the outer tube up and down several strokes
until no more bubbles come out from the oil.
• Keep the front fork vertically and wait 5 – 6 minutes.
NOTE:
* Always keep oil level over the cartridge top end, or air may
enter the cartridge during this procedure.
* Take extreme care so as to pump out air completely.
• Hold the front fork vertically and adjust fork oil level with the
special tool.
NOTE:
When adjusting the fork oil level, compress the outer tube fully
without the fork spring.
 Fork oil level: 153 mm (6.02 in)
 09943-74111: Front fork oil level gauge

Remove the gearbox breather, should be on the top of the box. Pour it in there. Might take a bit of time though. Make sure you're putting the right amount.

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Get yourself a Honda, Suzuki, Yamaha, Kawasaki. This will take care of the Harley shake, that is what Harleys do.

Stop riding the bike until you change the gearbox oil and oil filter. Yes, the oil is probably the problem. I am somewhat surprised that the pistons haven't seized yet. On the bottom or lower side of the gear box there will be an oil drain plug. Drain the oil and put in new oil. The oil fill hole is probably on the right side cover near the rear of the casing. The plug will probably have a dip stick to measure the oil level. Put in 10w40 motor oil. Ride the bike for a few hours then drain the oil to help flush out the dirt and grit built up in the gearbox. Put in fresh oil and a new oil filter if the bike has a filter.

Your bike is 18 years old and parts can be difficult to find. Your bike probably has an oil filter. If so, it desperately needs a new one. Google " vintage suzuki motorcycle parts " . I can not find a free service manual online. You may be able to buy one at www.repairmanuals.com