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Hi All,

I have a mark 2 GS300 Sport, I have had it for over 3 years and always used a fully synthetic 05w-40 oil, It has now 152k miles and I have had it since 88. I do alot of motoway mileage (about 99% - 1% in town). As such, I thought it may be better if I used a thicker fully syn. Halfords now do a 05w-50 fully syn (I have always used their 05w-40) as I do alot of motorway driving, I thought of swapping to the 05w-50 from 05w-40.

Will this be ok for my high mileage care or should I stay with a thinner 05w-40 synthetic or due to the mileage go for a semi-synthic if so which one????

Any advice greatly appreciated.


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If you have had no problems whilst covering 64k miles why would you think it might be better to change your oil? Having said that, I can't see what harm you could do by sticking with fully synthetic but changing to the 05w-50.

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i thought the 5-50's were for high high performance turbo cars ... i thought the 50 bit was the temperature range and the thickness...

i.e. a 5-30 oil would reach optimum working temperatures quicker at lower temps where as a 5-50 oil would remain thicker for longer untill it had reached the higher temp..

I know that some of the guys running silly power skylines were using 5-50 oil as they were killing the normal stuff too quick..

So is it possible that using such a heavy use oil at these current low temps might actualy do more harm than good?

I have already noticed that my car does not feel the same after doing a change from 5-30 oil to a 10-40 oil..

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I have always used Mobil 1, 0-40 and have never had any problems, its a very high spec oil, the thinner the oil the better for the engine, fully synth will give full protection at whatever consistency it is. the probelm with using fully synth on "poorer quality" engines is that the thinner oil will leak out, bypassing seals and gaskets,lexus engine is extremely well engineered and should not suffer from this problem.

"extract from the internet"

In short, if you see an expression such as 10W-40, the oil is a multigrade which simply means that the oil falls into 2 viscosity grades, in this case 10W and 40.

This is made possible by the inclusion of a polymer, a component which slows down the rate of thinning as the oil warms up and slows down the rate of thickening as the oil cools down.

Multigrades were first developed some 50 years ago to avoid the routine of using a thinner oil in winter and a thicker oil in summer.

For a 10w-40 to attain the specification target a 10W (W = Winter please note!) the oil must have a certain maximum viscosity at low temperature. The actual viscosity and the temperature vary with the viscosity grade but in all cases the lower the number, the thinner the oil. For example a 5W oil is thinner than a 10W oil at temperatures encountered in UK winter conditions.

This is important because a thinner oil will circulate faster on cold start, affording better engine protection and therefore lower long term wear!

For a 10w-40 to attain the other specification target a “40” oil must fall within certain limits at 100 degC. In this case the temperature target does not vary with the viscosity grade, if there is no "W" the measuring temperature is always 100degC.

Again the lower the number the thinner the oil, a “30” oil is thinner than a “40” oil at 100 degC, which is typical of maximum bulk oil temperatures in an operating engine.

Engine makers are, of course, very well aware of this and specify oils according to engine design features, oil pump capacities, manufacturing tolerances, ambient temperature conditions etc. It is important to follow these guidelines, they are important and are stipulated for good reasons.

Finally, if the engine has been modified or is used in stressed conditions, the operating conditions may well be outside the original design envelope. The stress on the oil caused by increased maximum revs, power output and temperature may require that an oil of a different type and viscosity grade would be required.

These examples show viscosities at different temperatures:

Grade................0degC............10degC...... .........40degC...............100degC

0w-40...............665cst.............354cst........ ..........82cst................14cst

5w-40...............842cst............ 430cst..................91cst................14cst

10w-40.............874cst.............440cst .................91cst................14cst

15w-40...........1260cst.............595cst........... ......107cst................14cst

i buy mobile one for around £24.00 of 4L, Halfords sell it for £34.00, so shop around.

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If you really want some proper advice about your choices of engine oil, I would recommend you look up the "oil man". Has he been on this forum? If not he shouldnt be hard to find as he is on every other car owners forum ive used. He gives great advice and knows what he's talking about. If you have no luck, i know for sure he is a member on

Hope that helps

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Thanks for the advice,

My thoughts were that if I change from a 5w-40 to a 5w-50 it will provide better protection for motorway driving (as 99% of my driving is on the motorways and then coming to a standstill -M25 usually). Would I be correct in this assumption?

In relation to seals. I recently did have to change the crankshaft seal (£450) as it had a leak, which I am told is usual for a high mileage car, as it has done 154K.

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