i have issue that my mechanic says may not be a real issue. when my 1999 XK8 warms up fully I start to have low oil pressure on gauge and low oil pressure light comes on when at idle in gear. when I put it in neutral or park the gauge reads fine and the light goes off. runs great otherwise but this bothers me despite my mechanics assurances. I only have 57000 miles on it

 

You can replace the low oil pressure switch near the oil filter...it is easy to do and inexpensive BUT another common cause is bits of plastic debris from the timing chain tensioners and guides that have accumulated in the oil sump.

If you have already upgraded your chain tensioners, then just drop the oil pan and inspect and clean the sump of debris.

If you have not upgraded your tensioners, consider this a fortunate warning and have that done as soon as possible and also clean your sump. Failure to upgrade your tensioners (in particular the secondary tensioners) will likely lead to early demise of your engine.

Issue can also be with the oil pump, but it is probably less likely that the above.

 

I have to 2nd WhiteXKR's info and add this. If oil pressure was considered to not be an issue at low RPM's at idle in gear or out they wouldn't bother with a warning light in those situations. It's telling you that there's a problem that needs your attention and the sooner the better. They don't call them idiot lights for nothing.

 

Ditto above comments, there was a recent thread on oil type/grade that should be used. Some very good thoughts were posted, do a search and look for one of the past 2 weeks, do you know what oil you have? 5-30 / 10-30 ?? Having a pre 2000 car it's going to have some wear and might just need a bit of help.
Good luck

 

Just out of curiosity, what is your idle speed in gear? Is it below ~650 rpm? Does the idle speed go up above that when you put it in park?

+1 on having it checked. That warning light is coming on for a reason.

And on whitexkr's comment, replacing the oil pressure switch on mine cured the problem.

 

Well , let me tell you my tale of woe with the same problem. My 97 xk8 has 104K on it and runs great. Had the tensioners done. A month later I noticed the same issue you have. I replaced the sending unit. Not an aftermarket. Still had the problem. Had my mechanic check pressure with a guage and realized it was not the sending unit. Had my mechanic pull the pan and see if the pick up was clogged. Nope. Decided that maybe we should pull a rod cap and inspect a bearing. Well there it was. Just enough wear too cause the issue. Pulled all caps and problem was the same. Good news was the crank was still pristine, no nicks or wear marks. Replaced all the bearings and oil pump since we were in there. Drove her for a while and pressure is perfect now. Only one other problem, the higher oil pressure blew out the rear main seal. Nice. Now we pull the tranny and replace that. Your engine has much lower mileage so I am hopeful you won't have my issue. But never rule these things out. good luck.