L322 v8 niggles

Hi all i have a 2002 4.4 v8 l322 and have a few niggles that really bug me so much it makes me want to sell it.

First one is the "check coolant level" comes on but the level is ok? Ive found this to be the case more so when the car is driven hard?? Then after a while once its come on when the car selects first gear you feel it go in and the revs jump to 1k as it pops in??

I have read up on the coolant system that it runs and it got a bypass system and the secondary purpose for the coolant system is to cool the transmission fluid which is why i come to think they are connected.

I have downloaded the workshop manual and its got the flow diagrams for the coolant system in there and the only thing i can think is the thermostat is not opening correctly so the water in expanding a little too much bringing the "check coolant level" on and its not able to cool the transmission fluid down hence the noticable gear change down to first (all others are smooth as silk) could it be keeping the right temp by just going round the bypass system?? But i thought if your thermostat was faulty temp gauge would go really high but is sits perfect in the middle,

Any suggestion????????????????????????????????

Second is before i had my car serviced (local LR place) everything was fine, i had inspection 2 service done where oil was replaced with 5/30 Q8 oil after 500 miles the engine started tapping just for couple of seconds when the engine was started, so run garage and we decided to put in 10/40 Q8 oil but again after a bit its started again (did it tonight first time) so it must be wrong oil for car as it never did annything like this before (when it was doing it first time is wasnt everytime you started it just every now and then).

Any suggestion ???????????????????????????????????

Apart from these i love the car but it just annoys hell out of me.

Thanks guys

I don't know what it is but i don't think its the oil, oil will quieten thing for sure but it wont cause a noise, if the grade is correct, then thats it, it lubricates and cools things down (well transfers the heat) but it wont make something knock or a tapping

Ive run the car for over a year and its been all ok no noise nothing till the oil was changed on the service.

I dont mean it does it all the time when engine started but every now and then for couple of seconds on start up like there no oil on cams.

I had a similar thing but was on my nova 2.0 16v i put magnatec oil in and it solved it because it sticked to everything no just run back into sump.