1995blackttopta

i'm looking for a slightly thicker oil because when my car warms up, the oil gauge is reading about 10psi @ hot idle & 500-600 rpm in gear.

but, if the air temp outside is above say 80 or so, the oil pressure will driop to 0-5 psi at hot idle (check gauge light comes on, gauge is right above 0) i'm pretty sure this is just a gauge thing because the last time i had this happen i sat in idling traffic for around 20 minutes with the gauge on 0. revving the motor up a bit would cause the light to go away. now that its cooled off outside it enver does this but the oil gauge does read halfway between the red line and the 20psi mark which is a bit low?

i tried changing the oil sender unit from one from AZ and it actually made the problem worse...once the car warmed up, every time i would pull to a stop it would drop to 0 and "check gauges" would come on. i only left that sensor in there for a day or so before switching back to the old one (which was only a year old). will 10w30 raise my oil pressure on a hot idle from 5w30 or do i need to go to 10w40? i'm running mobile 1 5W30 FWIW

Green96Z

The first number is for cold viscosity, the second number is for warm. So 10W-40 would be thicker than 10W-30 at operating temp. But all you're really doing is covering up an underlying problem.

Try using a mechanical oil pressure guage instead of the sending unit. I wouldn't use that AZ replacement again anyways. If you replace it, go with AC Delco.

1995blackttopta

yeah i returned the POS az one and am using the original one. i probably should check it with a real gauge since this whole check gauges thing started after the shop that rebuilt my trans last november broke the oil sending unit and had to replace it. i stored the car all winter, and other than some oil leaks and running a little bit low on oil once (low oil came on after i stopped at a gas station, i immediatly shut the car down and added 2 qts) and then had the intake manifold gasket valve cover gaskets and oil pan gasket changed so i wouldnt ever be low on oil again.. but the check gauges thing was happening before i had those gaskets changed and that didn't seem to fix it although i know i'm not low on oil. i would think for as many times and as long as that gauge was misreading pressure once while in indy if i had a real problem i'd have some obvious engine damage, which i do not. i was in a line of cars getting on the track at indy and every time i came to a stop the check gauge light would come on with the gauge in the red. i'd shift to neutrol and rev it a little and it would be fine. this was on a very warm day (85+) and now that its cooler it hasn't shown the CG light since i tried changing the sender back in early october and when i saw that one was defective since it woul hold no oil pressure whatsoever at idle once warm, i changed it back. now i get pressure, but its halfway between the red and the 20 mark so 10 or so which i know is normal, but why does it read lower when the air temp is over 85 or so? thats what makes me think the oil is thinning out too much.

Green96Z

Heat makes liquids expand. Expansion increases pressure. I would definitely hook up a mechanical gauge so that you can be sure you're not damaging your engine.

1995blackttopta

expansion increases pressure? don't you mean it decreases it? everyone knows that LT1's have higher oil pressure when the engine is cold than warm

bombebomb

Heat makes it expand, but it makes it thinner cauesing lower oil pressure. Its like letting a 500psi bottle open into the normal air, pressure went from 500 to basically zero (or whatever) because that large amount of air is no longer in a small container.

To the op 10w, or 5w, or 0w whatever it may be is the cold viscosity of oil. The second number is the operateing temp vicvosity. So 0w50 and 20w50 are the same thickness during operateing temp.

1995blackttopta

so what is the right oil to run? 10w40? ive heard lots of people say that 5w30 is too thin for 100k+ LT1's

i have 123k FWIW

SnakeOiler

5w30 is fine to run. You should have no problems with that.

But back to your original issue. As mentioned, get a real guage to check the fact if you have correct oil pressure and go from there.

1995blackttopta

yeah i probably should, i tried a different OSU and it made it worse, so i went back to the old one (which was only a year old) i dont have any problem unless the air temp is over a certain degree so i thought maybe my tolerances have loosened enough to require a thicker oil. considering i was in a 20 minute stop and go line to get onto the indy raceway track back in september and saw my check gauges like every time i was stopped, and ive put something like 1000-1500 mi on the car since then, i doubt i actually had that low of an oil pressure. especially when during the time i changed out the gauge to test it, the current gauge was reading 10 psi or so at hot idle and swapping the gauge made it go to 0 every time i was under 20mph with my foot off the gas, and putting the other sensor back in stopped this, that there is definately something weird there.. maybe with the old sensor back in there (which was only a year old FWIW) its closer to normal.. i wonder if the tranny shop damaged the wires or something since the problems seemed to start after that

Green96Z

You can't look at it like that. The engine oiling system is a closed system, which is why there is pressure in the first place. If you heat up a bottle of soda, eventually it will explode because of thermal expansion, which builds up the pressure in the bottle (excites the molecules causing them to speed up and bang against the sides of the bottle harder), and causes it to explode.

pro94lt

I switched to 20w-50 and love it. 117k miles

1995blackttopta

isn't 20W50 like cement? i'm looking for something a little thicker, not something thats gonna clog up my engine

bombebomb

Yeah, I was trying to think of a better way to explane but, between not being able to, and scratching my head at your avatar I went brain dead.

To the op, I used 10w 30 synthetic in my car that had 120k+ miles on it, but oil is one of them touchy subjects. The oil cap said to use 5w 30 is thats worth anything to you.

SnakeOiler

You don't need anything thicker. You used two different oil sending units and have two different results - obviously it's not the oil.

It's either a bad sending unit, bad gauge, debris in the oil sending unit and/or fittings, something internal or a combination of any of these. Thicker oil will not solve your problem (it might mask it, though)

Injuneer

The engine oiling system is NOT a closed system, and the thermal expansion of the oil is not a significant factor in the development of oil pressure. The oil pump is a fixed displacement (volume) device, so it pumps the same volume of oil per rotation whether the oil is hot or cold. The pressure at the sensing point is developed by the resistance to flow in the various channels that it has to flow through, downstream of the sensing point. The resistance to flow in the flow channels obviously is a function of the oil viscosity, which does vary with temperature. Eventually, the oil pressure reaches "0" when it exits, for example, the push rods.