15/40 Oil OK?

[Mj17]

Is 15/40 engine oil OK in the Triumph straight-6?

The oil pressure guage feed pipe popped of the T-piece on the way to work this morning and sprayed the contents of sump around the engine bay.

The closest to 20/50 the petrol station I dived in to had was 15/40 and that seems to have been OK for the 80 mile commute.

Question is - should I try to change it ASAP or is it close-engough to 20/50 that it will be OK till the next scheduled oil change?

Thanks

[AlanChatterton]

Change it now.................!!!

15/40 is just not thick enough for these old engines.

Change it to 20/50 now, don't wait!

[Slimboyfat]

Don't talk poop Chatterton! Been using 5W-40 in our 'old' Triumph engines since Mobil introduced their Mobil 1 20 odd years ago. It's been a god send with engines running to phenomenal mileages as a result. Before that I was a Castrol GTX 20/50 man (inherited from my father who had used the stuff). Back in the 70's, early 80's when I still used GTX I thought it was normal for the oil to turn to the consistency of dishwater after a good run up the motorway in the TR5, or after a couple of runs at Santa Pod. Then a certain Mr John Kipping showed me the light (synthetic oil's) and my engines have lasted at least 3 times longer than in the bad old days when I used mineral multigrades.

[DJB_Harvey]

Good engines can certainly take advantage of modern oils..certainly 15/40 is fine as for synthetics ive been using mobil 1 in my PI for years now and thats 0/40 ! however old /unknown quantity engines just bung in a cheap 20/50 and change it regular....also worth looking at Millers oils they do a fine range and i know a lot of the TR race cars use there products. I believe Witor swears by halfords 5/40 synthetic in his own car so it must be good !

[Deleted User]

Witors car hasnt been on the road for years now tho hasn't it?

[Dizzy]

Thinner oils circulate more easily and give easier cold-starting, which are good points of course.  But they are more susceptible to shear at high temp/high load and they are likely to drop the oil pressure slightly which might not be acceptable in a worn engine.

I always use 20W/50 as Triumph built the engine with this in mind but I see no problem with leaving the 15W/40 in until the next oil change (3000 miles maximum if using mineral oil) so long as it's a standard-spec engine that's not too worn.  If we were about to enter a very hot summer, 15W/40 might be pushing it a bit - but I don't think we need worry about that just now!

[Clive]

Dizzy wrote:

I always use 20W/50 as Triumph built the engine with this in mind

I really doubt this. I think it was the most modern oil available.
The lower the first number the better. As to the 50 when hot, I suspect that a modern 40 synthetic will cope much better with extremes of temp (ie high) than some of the 50 weight oils around.
However, for limited mileage I suspect a 20/50 will do an admirable job. A good quality fully synthetic will way outlast it though, be more efficient and result in less wear. Can't be bad!

[shedmonkey]

tend to agree-older worn engine use some 20/50 glug-I use Valvoline 20/50 racing change often
Newer known good engine use synthetic -dont think I have ever had one of those!!

[shedmonkey]

sorry -as for the orginal question 15/40 should be fine for 3,000 miles or so I would have thought?

[Greeks]

I did it once on BOB's old engine ... wouldn't touch it ever again. I'm sure that's when the top end started to make noises (80,000 miles). If it was me i'd change it asap regardless of what anyone says.

[ferny]

Ok, if fully synthetic is good for a new engine and mineral is good for a worn one, what should I use in my slightly worn engine which won't need changing so often? I on my third lot of fresh oil this year already... I've just changed it before the RBRR and I'll need to change it again shortly after that too!

[Slimboyfat]

ferny wrote:

Ok, if fully synthetic is good for a new engine and mineral is good for a worn one, !

Our Chicane has been using Halfords fully synthetic (couldn't afford to keep putting Mobil 1 in it!) for the best part of 10 years. It used to do about 20K miles a year, but more recently it's been around 15K. Started using the stuff when we acquired the car with it showing 95,000 kms (Chicanes have kph clocks) but I was suspicious that it had already been round the clock once before. It's been around the clock twice more since then (miles not kph), and all on the same engine. Maintenance has been limited to pulling the sump off when we first got it, cleaning the crud out, and checking the shells, and that's it!

[Dizzy]

Clive, I don't know how - datewise - 20W/50 ties up with the design/build of our engines but Triumph's tests must have shown that this grade was correct for the bearing clearances etc of the engine.  A very thin oil may not give the required boundary ('unpressurised') lubrication when starting the engine, and hydrodynamic ('self-pressurising') lubrication may also be affected as oil viscosity plays an important role in this as well.

Also, a thin oil will more easily find its way out via worn seals - and even via unworn old-technology seals like ours - as well as down the valve guides etc.  For these reasons alone I cannot agree with you that "the lower the first number the better" .  Oils have vastly improved over the years but that doesn't change the need to match the viscosity to the engine design.