What kind and how much oil should i put in the crank case of my 1975 suzuki ts 185 and would to thin oil cause it to lose compression?

Ts 185 is a two stroke, the crank case oil is actually only the gearbox oil, the volumne is caste onto the side cover(like 750cc).near the filler cap.
The engine oil is mixed with the fuel and is pumped in from the oil tank under the seat.
The wrong oil or no oil in the tank under the seat can cause it to lose compression. and cause engine damage.
Visit a bike shop and get motorcycle gear oil20w50 for gearbox. and two stroke motorcycle engine oil for the oil tank.
As it is quite old someone may have removed the oil tank, thinking they will make it go faster or save weight or another stupid reason, if this is so you need to mix the two stroke oil with the petrol when you fill the tank at approx 25-1 ( 25 litres petrol mixed with 1 litre of oil)