My EU import WRX 03 is due for its service soon. Dealer says they use Subaru recommended Castrol 5/30 oil, didn't tell me amymore than that.

Many threads say to use Castrol RS 10/60, I rang Castrol UK to check what I should use. They said 10/60 is a bit too thick compared to what Subaru recommend and that RS 0/40 would give me lower temp protection but I'm not convinced after reading 0/40 Mobil 1 is too thin.

Now confused!

Andy.

Don't know much about the '03 engine, but for the classic shape car anything less than 10 was considered a wee bit thin, some even use 15, I would use Castrol formula RS 10w60, but do a search and read the posts, I'm sure there are loads

castrol magnatec and never had a problem with it

Dont use a 0 grade.

Castrol RS 10w60 is fine as is Motul etc. I personally think 5w is too thin also.

MB

My 1998 Scooby turbo owners manual states in black and white a Caution that SAE 5W30 is NOT recommended for sustained high speed driving.

I can only hope it is regarded as suitable for 03 models, even then 5W30 goes against advice on this site...

I’ve been reading oil threads with some perplexity for a while and feel its time that I made a controversial contribution. My Sti8 handbook clearly states that a 5W-30 oil is preferred. I am currently using Mobil 1 0W-40 (I can already hear the sharp breath intakes) and my concern is it might be too thick. I’ve trawled up the following viscosity data to support my arguments. Remember that the W SAE rating is measured at 0°F (-17°C) and the last time I was that cold was 2/3 the way up Mt Everest. Please note the curiosity that the 0W-40 is thicker at 40°C than the 5w-30 and really only gets thinner below 20°C on the way down to the 0W rating at 0°F.

Viscosity in centipoises at 40°C 100°C
5W-30 56 10
0W-40 80 14
15W-50 125 17
SAE 60 - 24

If for convenience we assume a highish operating oil temperature of 100°C then the preferred viscosity is 10 cP. My argument is this: if I use a 0W-40 oil then it is thicker at operating temperature than the handbook recommendation at 13 cP with 50 and 60 oils much higher still. I think the Sti8 VVT uses oil pressure to operate the valve mechanism so can this cause a problem? (I’ve no idea).

Even if you are a fan of SAE60 oil you only get 24 cP at 100°C so how can a 0W or 5W oil be too thin in cold operation when it is significantly more viscous cold than the 60 oil is when it is hot. Starting and cold operation has to be better with a thin an oil as possible.

Thick oil will reduce engine power and slow down turbo spool up response. Considering a majority of contributors on SN go to a lot of trouble to extract as much power as possible from their engines why not go for a thin synthetic rather than a thick one. I make this case for the newer engines where the manufacturers recommendation is clear, it may be that with the close tolerances in modern engines that thick oil can actually do harm by not circulating properly.

my engine runs at 105 degrees C cruising on the motorway, around town it runs around 100 degrees C. This is normal for an Impreza.

If you go to the Oficial STi website, they sell Motul 15/50 and no other oil.
http://www.subaru-sti.co.jp/parts/im...image/3-11.jpg

0/40 is too thin, a 0 grade is never recomended, lowest in any literature is 5. The latest manuals state "NOTE:
If the vehicle is used in areas with very high temperatures or for other heavy duty applications, the following viscosity oils may be used: API classification: SL or SJ or SH SAE Viscosity No.: 30, 40, 10W-50, 20W-40, 20W-50."

Castrol RS10/60 is a very good oil, i use that or Motul 15/50.