Why doesn't Marty use the other DeLorean?

You do have a point in that the fuel could have been syphoned. However, it's more likely that there just wasn't any. When the doc gets back to 1885 with broken time circuits, he first tries to fix it. When he realizes it can't be fixed, he buries the car in the mine after first draining it of all fluids.

While it is unclear if the car could be dug up, it wouldn't have had any fuel. The doc would have had to keep all fluids that he drained from the car. You could argue that some gasoline could always come in handy, but as the doc mentions in his letter, he is concerned about the space time continuum. Keeping it around in his shop would be dangerous (what if someone found it?) and he wasn't going to use it in a car as he specifically forbids Marty to rescue him. To avoid disrupting space-time, the simplest thing would have been to dispose of all fuel.

The official explanation can be found here. Apparently the time paradox for going back into the mine was too dangerous too go back in. However, Doc could have still drained it and kept it in his shop. Either he didn't want to keep it around (for the danger of discovery) or he didn't see the point in keeping it.

If Marty & Doc would have dug up the 1885 Delorean, then it wouldn't have been in the cave in 1955. Therefore the Marty in 1885 would possibly cease to exist.

As noted by Doc's letter to Marty, Doc has been in 1885 for 8 months. This is crucial to figure out why the gas, how ever much is left, would not be useful. Gas tends to have a shelf life of 3 months. 8 months is more than twice that. And that's only in ideal conditions. Since Doc likely knew enough to prep the damaged DeLorean for long term storage, which involves in draining all fluids, and that 1955 Doc specifically told Marty at the drive in that he filled it with gas, we can assume it had no 70 year old gas inside. Doc, had he kept it, would have needed a container, and 1885 would not have any quality, gas grade containers (air tight, opaque, low permability).

Gas will go stale within months. And that's modern day Gas, who knows what 1955 gas shelf life is like. The more space in a tank, the more space it has to evaporate. Condensation will add water to the fuel ruining it. Temperature swings will as well. As we know, 1885 had no temperature control to speak of. The older the gas, the less likely it will ignite. Diesel would last longer as it does not need a spark to ignite, just compression.

In short, even if Doc had saved the gas, eight months and imperfect storage would have made it useless by the time Marty showed up.

It's simple. If they would dig up the first time machine it had been siting in the mine for 6 months, when doc got there in 1885. when Marty got there in 1885, the replacement parts to fix it for 1955 would not be available until 1985

Major plot hole they didn't even need the gas they now had a working time circuit that they could place on the DeLorean that doc arrived

They blew up the engine when they first tried to get it running on whiskey, therefor not having enough time to repair the engine before doc's murder and if they had switched engine parts ir still would have changed space time continuum because the engine parts would not have been availble until the early 80's for the delorean

When Marty went back to 1885 Doc had already prepared the Delorean for a 70 year hybrination. This would have involved draining the fluids, especially gasoline. What good would gasoline be to Doc in 1885 when nothing runs on gasoline? I doubt he would have kept it. The 1885 Delorean was never shown because if it was disturbed or moved, it would not be available for Marty in 1955. As soon as it was moved or changed in any way, Marty and the 1955 Delorean would fade from 1885 existence. The only way the 1885 Delorean would be helpful is if Marty got there before Doc drained the fluids. Then they could have filled the 1955 car with gas from the 1885 car, then prepared the car for hibernation exactly as Doc had done originally. It would be a paradox if even the slightest thing was done differently to the 1885 Delorean so it would have been best to stay away from it completely.

Because if he used the gas in the car in 1885 they wont be able to use the car in 1955 cause theres no gas because they used it in the past there for doc is dead