Hi, I ran my car for the first time this weekend driving from London to Newark and back. On the way there (in rain) the oil pressure dropped to 40-50lbs but on the way back it fell to 30-40. The engine temp was cool at about 80 degrees. The only other thing is that I could have the timing and the fueling wrong as I haven't really sorted my MSnSE ECU. Also, the oil is new but not exactly top quality. No oil is being burnt and I have an oil cooler.

Any ideas on what this is about?

change your engine oil and put a good spec oil in, its not uncommon to see this in engines that have been stood or havent had a good oil change in a while as oil is hygroscopic like brake fulid, it absorbs water from the air.

wouldn't the water come back out of the as steam in the breather?

yes it would but that was more of a secondary explenation, i see oil pressure problems on a daily basis at work where the oil is either too old of too worn out to do the job, remember a good oil has a life span be it in KM or Hours running. and the harder an engine is working the harder the oil is working

Could also be the grade of Oil . It could be getting to thin when hot and therefore pump cannot maintain pressure. Check the oil spec is correct for the engine type.

Change it for a decent spec oil of the correct grade . Vauxhall oil seems to be OK .Not sure what the correct pressure should be but it may be worth checking.