[Closed] Explain engine oil viscosity

You're right about the numbers, but the viscosity will be stable after the engine reaches working temp so won't be changing any after 200 miles.

So that won't be it, I meant to add!

Knickers.
Any idea what would cause lack of compression (that's what it feels like)?
The earliest I've noticed a lack of power at top end is about after 90 miles of motorway driving.
I'm begining to wonder if it's all in my head.

Have you tried changing the fuel filter?

It could be getting clogged up and losing its efficiency after that sort of mileage, also it could be something like the vents in the fuel tank being blocked, creating a vacuum in the tank.

I'm guessing its a diesel, not the v8. It its worn that should become apparent almost as soon as it warms up and you will get oil loss and smoke from the exhaust. I wouldn't change to a thicker oil unless it really is worn. The Tdi diesel is a robust unit if its looked after, plenty do 300,000 plus with no trouble, mine has 205,000 on it and doesn't burn a drop.

Blocked fuel filter or tank breather sounds more like it. If you stop and take the filler cap off after a run, is it hard to shift at first, then you get a rush of air into the tank

You could try asking on www.difflock.com - helpful bunch on there

You engine performance issue is not connected to your oil. If an oil issue was causing your car to be noticeably down on power/torque, then you've some *serious* mechanical issues looming within the next few revolutions!

correct me if i am wrong here but the lower the numbers the thicker the oil, i used to have to put ep70 (lower if i could find it) in my cog box of my old P{ug and the current citroen so that when warm it doesnt crunch when down changing at revs, - warn synchro's,
a 5/40 is a thicker viscosity over a 10/40,

have a nose on here, they are very good and will answer normally very quickly!
http://www.opieoils.co.uk/

lots of information on the site too in tyhe techy bits!

5/40 is a thicker viscosity over a 10/40

No the higher the numbers the higher the viscosity. The W figure is related to the cold cranking viscosity. The lower the W number then the thinner the oil is at low i.e. sub zero temperatures and the easier the engine will turn over from start in cold climates. The other number is related to the kinematic viscosity at 100°C i.e the temperature of the crankcase. If you are after good fuel economy use the lowest viscosity oil recommended for you engine. Do not put a low viscosity oil in your engine if its not recommended as you will probably damage the cam, cylinder liners or journal bearings.

Oldgit the oil is not the cause of your problem although putting a thick oil like a 15/40 in the engine won't do it any harm and may well be what's specified for the engine in any case.

Timmo, no for engine oils the higher the number the higher the viscosity. The 2 numbers refer to multigrade oils with the equivalent viscosities of the oil in summer and winter (before multigrades people used to have to change oil for the season). 5/40 is thinner than 10/40 within the appropriate temperature range.

(edit)

see for example

http://www.driverstechnologyassociation.co.uk/oils.htm

5W-40 is only less viscous than a 10W-40 when the engine has just started. When the engine is at temperature both oils will have the same viscosity.

mate what engine have you got in it, as said nowt to do with oil, mechanical problem.but i need to know what engine you got.

thanks guys! one of those things i can never get my head around! always been under the impression it was lower and thicker!
many thanks!!

Hi
It's 1996 TDi with 177K on the clock. I've just done the cam belt, fuel filter and lift pump.
Not losing a drop of oil and getting a good 33/35MPG with Excellium diesel.

To the OP - as said, forget the oil - not your problem. (some) Reasons to be down on power after a period driving...
Sensor faults
Overheating (but possibly not showing up on needle)
Intercooler heatsoak
Plastic hoses warming up and allowing some of your turbo pressure to be lost past clips or through small cracks.
Wastegate sticking slightly open on turbo (would make spool a lot slower).

hi mate , did you have the lack of power before or after you did cam belt.cheers

Any idea what would cause lack of compression (that's what it feels like)?
The earliest I've noticed a lack of power at top end is about after 90 miles of motorway driving.
I'm begining to wonder if it's all in my head.

They say that cars start to resemble their owners after a while.