Got the old bird out the garage at the weekend, give her a wash, but noticed the day after, the front brake lever is not engaging sll the way out with the plunger, have to manually flick for brakes to unlock, I’ve checked the oil level, it’s half way up the site glass, looks a little dirty to be honest, question I’m asking is what oil does it use, I’m seeing dot3-4

And is it possible it could have got water in somehow? Was working fine when I got it out.

That's easy; it doesn't take oil it uses brake fluid which absorbs water out of the atmosphere turning your brake fluid into something very toxic nasty. You need to do what is called a flush of the old brake fluid and replace it with fresh fluid without introducing any air into the hydraulic system. After that you will know if you require new rubber parts in the brake master cylinder or not.

... water content in old brake fluid will boil when the brake becomes hot with use creating the worlds tinniest steam engine inside your hydraulics, it will work right up until the brakes gets hot enough to boil the water creating air bubbles in your brake system which is bad.