About to change the oil in my Jaguar XK 4.2 engine, it has been rebuilt to much closer tollerances than original, and I'm trying to understand the rationale behind using 5W40 in place of 10W40.

As I understand it, 40 is how an SAE 40 Oil would perform at 100 Degrees. and 10 is how an SAE 10 would perform at some lower temperature.

By dropping to 5W40, at the lower temperature i.e. at cold start, the oil will be thinner, and circulate the finer tollerances better, but perform the same at the higher temperature.

Am I correct or completely mis understood what I've read a dozen times now....

Yup thats the way I understand it too

And then you have to decide, Synthetic or fully synthetic?

In my opinion a running in oil is required before switching to a high quality oil on a new engine. This will help the piston rings to "bed in" and prevent the bores becoming glazed, leading to oil consumption past the rings

thats how i see it too.i dont mean to hijack the thread but whats the score with synthetic,semi etc.most of the nissan forums recommend silkolene fully synthetic on my motor but its stupid expensive!is it really worth the cash?

Have a look on this link for opi oils.
The guy is on just about every car forum in the UK..goes by the name of Oilman.
A wealth of knowledge.

http://www.opieoils.co.uk/technicalinfo.aspx

not to forget the API index rating of the oil too.

I usually use a cheap grade oil (SF/CC) for the initial running in or just for an engine flush.

Afterwards I'll use a better quality like a SH/CD.

I dont put much store by the additives in the oil, I usually compare the API index and if they're the same, then I'll buy the cheaper oil.

[Edited on 7/4/10 by petercd]

Oil engine designs like the dear old XK the bottom end of which is related to pre-ww2 Jaguar and Standard engines don't do too well on fully synthetic mainly because they need some mineral oil in the mix to keep the oil seals from shrinking.

Also using a 5w/40 oil will tend to leak a bit more oil past the valve guides when cold or not running.

I would choose a 10w/40 part synthetic.
If you rush to your nearest Morrisions petrol station they may still have Havoline 10w/40 part synthetic at £7.48 for a full 5 litres.

Thanks all, some interesting views.

Will give morrisons a try....

Not strictly true, I believe the 10W is a different scale entirely to the 40 scale. Seem to remember reading it on one of the oils sites some time ago. They arent both on the SAE scale anyway.

Other than that, yes you are right, dropping the first number will give quicker circulation on cold start, but will give slightly lower oil pressure.

Most performance engines you should increase the hot weight by 10 and leave the cold weight the same if possible.

David

Not sure about the old XK engine but i know the modern ones (AJ6 onwards) dont like fully synthetic.

i thought the first bit was viscosity and the lower the thinner the oil....i think zeetecs like a thinner oil because of small oil gallerys and cold starting or something( i may be wrong)

my old landy v8 loved 10w40.........

Talk about old inline sixes, my Jeep Cherokee has 235,000 km. and has been of Mobil 1 Full synthetic since it's first oil change, no leaks goes 8000 km. between oil changes and does not burn off 1 lite in that time. Engine is AMC design from the 50's.