I should have said.
Jon
Castrol GTX 10W/30 is a stateside brew, Castrol in the UK do not even have a product sheet listed for.it. GTX 5W/30 is available in the UK to the garage trade but not generally available at retail.
It appears to be available at Asda and a few other places but I assume it is a rebadged Castrol Magnatec 5W/30 for Fords that is now re blended as an A1 A5
eddie
It’s available in lots of places…although the label says ‘Castrol Ireland’, it seems odd they don’t acknowledge its existance in the UK.
All the same, I’m changing from a 5W/30 to a 10W/30 and will report back how it changes the pressure behaviour…although I doubt it will be noticeable.
Hi Jon
Castrol do have a GTX 10W/30 spec sheet( middle East market). If this oil is the same then this is identical to your Mobil 5W/30 when hot but will be thicker (~20%) when cold…
R-b
Bet you wish you had the Lotus Cortina now looking at the prices.
Just to stir this age-old pot once again and to answer Rich M’s pondering, I removed the 10w40 fully synth and replaced for 5w30 part synth. I’ve gone from over 90lb/in when ice cold to 60lb/in from ice cold. Much better, and stays around 30lb/in more keenly too.
Better set of numbers IMO. Not to mention the fresh 10w40 came out black, so that flush of oil seems justified…
Thanks for the update Paul, good info 
Oil pressure always rises after oil and filter change. I work on a lot of generator/propulsion engines and is very noticable as the electronic pressure guages are accurate to 0.01 bar.
Reason being before the filter change, the oil is restricted due to build up in the filter, thus dropping the pressure (as most pressure readings are taking after the filter) with a new filter and less restriction the pressure seems to increase.
would be the opposite if pressure readings were taken before the filter
Now you realise why Mazda deleted the real oil pressure gauge after 1994* and went to a glorified on/off switch. OK, it saved a few pounds per car, but they were also fed up with queries from owners who didn’t understand why oil pressure would vary.
The Mk3 has a “fake” gauge which shows you what the car thinks the oil pressure should be, not what it actually is.
*you can fit the earlier sender and gauge fairly easily to a post-1994 Mk1.
Connelly 1888
What is the typical oil pressure of the engines before and after the oil change. I assume a small change in pressure readings
I have heard it said within reason if you do an oil change at a short interval that it is better not to change the filter as the improve they filtering performance after a while. I have never seen any quantative data to back this up.
I couldnt tell you what the mx5 running pressure is as I dont pick mine up till next week, but i guess under load would prob be between 3-4 bar at operating temp, 1.5-3 bar at idle.
The generator on our boat for instance
before an oil change: at start up about 4.8-5.2 bar, when at operating temp this’ll drop to between 3.8-4.2 bar depending on the load on the engine
after the oil change: at start up 5.5-6.2, then about 4.0-5.0 at operating temp, again depending on load
as you can see theres quite a big difference from a clean filter, almost 1 bar (or 14.7 psi in old money)
I never noticed a difference, but I was changing the oil every six months, not doing sludgy short trips, so the filter probably never got much crud in it.
The gauge seemed to be reasonably accurate (my sender and gauge were new parts ordered from the dealer and retrofitted to a 1997 Mk1) because the maximum pressure when driving at anything above idle, just after startup, corresponded to the relief valve opening pressure. I had a psi gauge (you could order either psi or kgf/cm2, maybe still can) and hot idle was about 25psi.
That is exactly what I would expect with a high quality oil and filter changed regularly.
I assume the commercial engines the other poster mentioned were not using oils that were similar to modern car engine oils or had some sort of slugdeing problem inherant in their design.
The commercial engines I refer to use a 15w/40 diesel engine oil of the best quality you can get, prob about £60/70 a gallon retail. and no, they dont possess ‘inherent design problems’ lol. they run 24 hours a day, every day using a crap quality diesel. no car engine would stand up to that sort of abuse without heavy sludging problems