+1 - me too on the JGR DT50 and I do have some scientific evidence as to why, since I do a UOA on all my vehicles at every oil change. Here are the results from my 928 UOAs since I've owned it.

Last century Porsche engines very much need high oil film strength engine oils if one wants as much long life as possible out of items that have no oil pressure keeping the metal parts away from each other , such as }

A ) valve guides ( air cooled 911 huge wear issue )
B ) Cam lobes & hydraulic lifter faces ( flat tappet design )= 944 / 928
C) Crankshaft thrust bearings
D) Camshaft chain sprockets ( 944 & 928 twin cam heads)
E) Alloy piston skirts against alloy cylinders
F) 911 air cooled rocker arms contact faces with cam lobes

All the above we have seen exceptional wear , meaning high wear , when these engines have been on low viscosity so called synthetic engine oils , and conversely we have little or no wear on the same types( & 3 times the milage) where they have been on the oil viscosities recommended by Porsche that is printed in the owners manual that came with the car new .

So on a last century Porsche }

911 air cooled ( all year types )
924
931
944
944S
944S2
928 ( 4.5L , 4.7L, 5.0L , 5.4L )
968
951

We see in the owners manual on the oil viscosity page , the oil chart showing ambient temp ranges

In the "Look at me " shaded section of this page under the heading of multi grade oils we see the first ambient temp range of = minus 10 deg cel to unlimited high ambient temp ( sounds like Australia to me ) , we then see the recommendation of a 20w-50 & 15w-50 ,
One does not see 5w-40 etc in this temp range

Note } in Porsche cars ( 928 , 911 , 944 etc ) up to about 1983 or so the ambient temp range for a 20w-50 was minus 15 deg cel to unlimited high temp range

Now lets go to engine oil film strength }

Question = how do I increase oil film strength with an engine oil on this planet in this universe ?

Answer = there are two basic ways

1) increase the oil viscosity ( say from a 5w-40 to a 20w-50) if both oils have the same amount of ZDDP

2) increase ZDDP or add Molybdenum ( in conjunction with ZDDP ),which enhances the effectiveness & life of the ZDDP

So if you add the two ( meaning 20w-50 & ZDDP ) = high oil film strength

Subnote } most if not all 20w-50 engine oils are OK in respect to their ZDDP amounts , because last century engine's & oils are not subject to this century emission protocols , most cars in this century use much lower viscosites ( engines designed for this)& to meet the emission protocols( lower levels of ZDDP & no Moly), meaning 0w-30 , 0w-40 , 5w-40 oils for this century cars are generally lower in their oil film strengths as compared to a 20w-50 with ZDDP , even though the replacement AW ( anti wear additive) of ZDDP ( read Boron ) has not been as useful in regards to increasing oil film strength as ZDDP was / is in last century viscosities.

There was even one famous oil company here in Australia that experimented with dropping the amount of ZDDP in their 15w-50 just over a decade ago & that was interesting , we saw quite a few worn cams and above parts mentioned above within 60,000Kms( on average) , Oh and by the way the same oil reverted back to a higher ZDDP ( it can be a bit of a mine field at times)

So you can see why I am not easy about recommending even a 15w-50 , even though it was probably a one off

The reason we can figure that easily is one only has to look at the advertising for this century racing oils for "this" century engines & oil companies are falling over each other to show how much ZDDP is in their racing oils , and yes they are not falling over each other to show how much more Boron or any other AW additive they have added , they just go on about ZDDP , so it must work over wise they would just say things like " we have added twice the amount of non ZDDP AW replacement like Boron etc " , well this is not happening , its ZDDP , yep last century solutions in this century racing oils ( who would have guessed that )

Now lets go to primary diesel engine oils , like Delvac etc , excellent oil for a diesel engine that is designed for that viscosity , however Mobil & Porsche do not recommend a diesel oil in a petrol powered engine , not last century & not in this century

Now why is that so ?
Answer = primary diesel oils are very high in detergents( compared to petrol powered oils) and these detergent packages tend to make engine oil foam too easily , so they then add silicon to reduce this tendency , but detergents attack the ZDDP , thats why primary diesel oils look as if they have very high ZDDP( new) , but as time goes by the ZDDP is damaged by the detergent packages .

Now back to the foaming , if a petrol engine from last century revs harder to make its max power as opposed to a diesel engine from last century ( like for like) this in quite normal , so in a big rig that cruises on the freeway at 1,800 RPM will give the oil way less tendency to foam as say a sports car / GT engine that is being revved to 6,500 RPM a lot .

This is where it gets interesting , foaming / air bubbles in the oil is the VERY last thing you need in a 944 / 951 or a 928 , because they have a very small flaw in their crank oil feed design , they starve no 2 con rod bearing ( 944 / 951 / 968 ) and 2& 6 con rod bearings on a 928 if the oil pressure drops to low.

How do we drop the oil pressure with a high core temp ?
Answer = use a 5w-40 oil ( in Australia) , then make it a diesel engine oil that will foam more easily & you are putting the bottom end at risk for not net gain what so ever

How do we reduce / eliminate this from happening ?
Answer = we consult the owners manual ( very easy stuff to understand)

Question = Do we see people destroy last century Porsche engines at the track when they are running even slightly lower viscosities ?

Answer = used to be very common , now getting better as people get better educated by reading their owners manual

Question = How do we eliminate this human tendency of wanting to use this century oils in last century Porsche engines .
Answer = Education .

Now onto oil pressure for a bit , we used to get ( every summer ) 928 ( all types) 944 / 951/968 turn up at our workshop with their oil pressure warning light glowing brightly at idle , naturally its a hot day & they have spent a few hours crawling in heavy traffic to get this high core temp ( no not coolant temp ) I mean core oil temp , and this is normally due to 5w-40 , we even get the odd one do it on a 10w-40 , so for these conditions the only thing that works is a 20w-50 ( never fails ( so long as the engine is not already badly damage by running around with a low vis oil

Remember , if the oil warning light is on at idle , then you can not go for full power because the con rod bearings are at severe risk at high revs , on a 20w-50 this will not occur .

I have been meaning to mention this for ages , but never never seem to get around to it , here are some basic in the field technical observations with Porsche car engines

A) Within the ambient temp rangers of minus 10 deg cel to unlimited high ambient temps( ie Australia) you often hear people say , thinner engine oil gets to the top of the engine quicker on cold engine start & / or builds up pressure quicker , so with this in mind its interesting to note some very interesting things like }

# Why is it that we normally only see things worn / pitted / stuffed that are not subjected to any oil pressure at all , like the following }

1) Cam lobes ( no oil pressure)
2) Hydraulic lifter faces ( no oil pressure)
3) Valve guides ( no oil pressure)
4) Cam chain sprockets( twin cam 944 / 928)( no oil pressure)
5) piston skirts ( no oil pressure)
6) Cylinders ( no oil pressure)
7) Thrust bearings (no/ extremely low oil pressure )

All the above rely on slash ( oil film strength only) except for the thrust bearings that are captive to the oil dumped from the mains


# Now to the components that we never see wear with ( 928/944), even on the wrong oil ( meaning even a way too thin/ low oil film strength oil )that get oil pressure }

1) Main bearings ( high pressure fed )( except thrust bearings)
2) Camshafts bearing journals ( high pressure fed)
3) Most of the conrod bearings ( not all if on a low viscosity oil)

So its the components that are subjected to oil film strength only protection that are the wearing items( normally ) , you simply can not give the components in the first list ( 1- 7 ) oil pressure , its just not possible ( for the price) .

It just shows how extremely important how oil film strength is

Five+ or so years ago I removed my twin cam cam covers for new cover seals ( I do not like to call them rocker covers because there is no rockers)from my 1986 928S ( M28/46) 5.0L , it was mid winter( I remember because I stayed back several nights to work on my 928 & it was freezing at that time of night) & my 928 was in the corner of the workshop waiting for me to put the covers back on with new seals ( when I had the time), it stayed there for a few weeks unstarted , what was interesting was that even after several weeks the cam lobes / lifters , chain sprockets were still wet/tacky with oil , nothing was dry to touch at all , then I thought I will do an experiment , I cranked my engine over with the twin cam covers off ( what a mess ), before even what seemed a second went by I had oil flinging / flying off the cams all over my guards & inner guards ( took ages to clean up ), this was on a 25w-50 oil at the time ( the old GP50 ), what struck me was the speed of the oil to the cams(journals) and then to flinging the stuff around , it all happened long before this engine would even fire up normally .

Conversely we have observed 928S4's & GTS's that are on a 5w-40 oil and a few days after the engine has been turned off , if one gets access to the cams they are noticeably dryer( less oily) to the touch , meaning they appear almost dry compared to the last century oils( 20w-50 etc) , so as one can see its not a one way street ( it works both ways ), this is because as the hot thinner vis oil drains off , it really drains off ( like kero) .

Remember , pitted cams ,lifter faces , cam chain sprockets( twin cam) ,piston skirts ,cylinders etc etc all happen with low oil film strength oils over 20,000 - 60,000kms with billions of engine revolutions , naturally the lower the oil film strength the quicker the wear and 99% happens during the most prevalent time ( hot/ normal running ).

So the cold start thing is true its just not exactly what one may think it means .

Regards
Bruce Buchanan
Buchanan Automotive