I'm finding it hard to believe that Castrol's 25w50 can get to 6.1 HTHS. What is its specific gravity from the spec sheet?
What engine are you going to put this oil in?
Is it roller or slider cam follower?
Does it have bearing clearances that are in the normal specified range for that engine?
Is this a racing or street engine?
What is the maximum expected oil temperature?
The Castrol oil has 880 ppm less ZDDP than the Pennzoil, but its 1000 ppm is still an effective amount, and would be OK in a roller cam engine. Also, the Titanium in the Castrol probably makes up for the lower ZDDP.
If the engine has normal bearing clearances, I don't see the need for an oil that is up to 6.1 HTHS. Is there any engine out there with "normal" bearing clearances (in the range of .001-.003") that needs more than 4.6 HTHS?
Based on the supply circumstances, I think the Castrol is probably a better choice, but I think its higher viscosity is a negative. There may be better oils for your application than either of these two.
Thanks for the responses so far gentlemen.
To answer your queries.
I don't have the advertised figures for the specific gravity of the Castrol Edge Titanium(with the advertised HTHSV of 6.1).
All I know is that the previous incarnation of the oil was called the Edge Sport, which has an advertised HTHSV of 5.4, with a KV of 212 @ 40 C and a KV of 21 @ 100 C.
No specific gravity is advertised.
The engine is a fully blueprinted and stroked Ford 351 Cleveland(approx 390 CID with high compression) and a fairly large roller cam that makes power to about 7500 rpm, with followers that are designed to bleed oil out of the lifter gallery in order to increase the splash feed of oil to the cam lobes/roller followers.
There's a high volume oil pump that's also blueprinted, with enlarged capacity drain back holes to the sump, and reliefs/modifications all through out the entire lube circuit of the engine.
The block cooling system is grouted up to level with the bottom of the welch plug holes in order to provide reinforcement to the thin wall castings of the cylinders.
There's a large capacity oil sump with windage tray/crank scraper and baffles around the pickup.
There's a large inline full flow oil cooler mounted in front of the radiator, ahead of a positive displacement oil priming pump which incorporates the remote oil filter mount. With a larger than normal full flow oil filter that's approximately 9" long.
The total oil capacity of the entire system is approx 10 litres.
As far as oil temps are concerned, it's still undetermined at this point what the operational oil temp actually is.
However I strongly suspect the bulk oil temps are currently a little low(and well under control) in comparison to a stocker.
There is a view to keeping the bulk oil temps to a target range of about 85-90 C.
A process of gathering the relevant operational temp information for the oil and coolant is on-going at this stage.
The coolant temps are extremely well under control/stable under all circumstances(even with a standard V8 radiator).
There is currently an option of easily going hotter with the thermostat opening temp in the near future, as I feel the coolant temp is also currently a little bit low for my liking.
There's plenty of scope to tailor the cooling system to achieve any target values as required.
I fully expect this will ultimately have a degree of a flow-on effect on the bulk oil temp.
IMO, the engine is way overkill/ridiculous in the street application it's being used for, but it's just a fun machine that's intended/built to be bulletproof.
So one could easily say that it's a racing engine for use on the street.
Bearing clearances are indeed within the normal specified range.
Like yourself, I suspect there is no legitimate requirement for an oil that's quite so thick at operating temps, and that the viscosity may very well be a negative.
I also concur with you on all other observations regarding the 2 different oils.
And I am tending to lean toward the Castrol in general.
Even though I'm not really a Castrol fan as previously stated in an earlier post.
However, in order to fill in some further background information.
The person who did the machining/blueprinting work is highly regarded in our country. But he harbours a bias toward mineral oil, and he is actually strongly "anti that synthetic water [censored]" to quote his words.
When an oil recommendation was requested of him, he simply stated "a 20w-50 would be good but a 25w-50 would be better".
And so it followed, that the mineral Pennzoil 25w-50 GT Racing oil was initially chosen as the initial fill/run-in oil out of the available viscosity options. It was also recommended by him over all else.
There's only the 2 brands available to us here in OZ in 25w-50 grade, and that's the Pennzoil and Castrol offerings.
Hence my enquiry to this forum, in order to gain some more information so a better informed decision could be made going forward.
The Pennzoil is not as readily available in our locality, and is quite a bit more expensive.
On balance, looking at the figures from our local lab, the Castrol offering easily appears to be more bang for the buck. So to speak.
I have not ruled out changing to a 20w-50 grade oil sometime in the future, and the choice is wide open in that respect, whether it be a mineral oil or full synthetic(which I am partial to anyway).
Oil pressure is very good under all operating conditions.
A straight SAE 60 full synthetic motorcycle oil was under consideration at one stage, but I had rejected it out of hand quite early in the process, on the basis that it would be much too thick at operating temps with a KV of 23.4 cSt at 100 C and a HTHSV of 7.08 with the bearing clearances on this engine.
There's absolutely no lag in achieving full oil pressure when the engine is started from cold, because the engine lube circuit is completely primed by the inline external oil priming pump prior to cranking.
A generous warm up period is also applied before driving off, with a view to limiting engine revs until the engine is fully up to temp.
I intend to stay with the 25w-50 grade mineral oil for now, unless something changes with regard to the availability or formulations in the future.
With the combination of high valve spring pressures and high lift of the cam, one can hear the followers well and truly hammering away on the cam at idle even with tight valve clearances.
Given the application and operating conditions, I think it may be in my best interests to go forward with the Castrol.
Thankyou for your careful considerations on this matter so far, and it's greatly appreciated.
My apologies for the long post.
I just wanted to provide as much background info as I could.
Any further input will also be greatly appreciated in case I've missed something, or you have any further ideas.