greybeard
Herd Master
Ya try turning it the other way?.....may have a left handed thread on it.....sometimes a glaze can form on those things and there can be a vacuum created.....just some of the things that I've faced a few times.....
Left threads..on a hydraulic filter?
No.
On modern filters, it's not the threads that are tight.
The 'o'ring or rubber-like (usually neoprene) gaskets swell up due to the heat from the hydraulic fluid and makes filters difficult to get off. The swollen membrane acts like a brake pad between the metal face of the filter and the metal base the filter seals against. Once the 'o'ring/gasket material swells, it never goes back down to origninal thickness, thereby ensuring a good seal.