Which oil now ?

Schrup · Schrup's Garage

I bought my car from a truck dealer whose kid drove the car for about 3k miles. The first thing I did was to get the oil changed. I had Castrol 20/50 put in. The car does not leak or burn any oil. I recently recieved an email from the long time PO who said he always ran synthetic in it. I've put about 1200 miles on it . An old aircraft mechanic told me I should switch back immediately. I thought I could wait till 3k then switch back without affecting the seals. I was hoping to get a consensus on whether I should wait or even switch. I don't mind the extra cost. TIA

island911 · 06-30-2003, 06:37 PM

I wouldn't worry about it, and I'm a 'synthetic snob."

The Castol is fine.

Did the old aircraft mechanic say you should switch back immediately, because of the the seals? (just curious)

Schrup · Schrup's Garage

Yea he seems to think the mix will have an adverse affect on the seals. But what about synthetic blends. My car has 105k & runs like a top. I thought maybe someone might know something I don't.

GSpreeman · GSpreeman's Garage

I just dropped in an oil change with Castrol 20-50 and I love it. My car's never run quieter. The PO had been using synthetic and as of yet I haven't seen any problems.

island911 · 06-30-2003, 07:00 PM

interesting. . . .Ya know how you can armor-all (or whatever) your plastic bits to help them live longer? . . well, motor oils may carry additives (plastisizers) for enhancing the life of the seals. (keep them soft and supple)

So, again, I think you'll be fine until your next oil change, but it is very interesting that the aero guy has such a strong opinion about the Castrol and the seals. Ask him for his opinion on synthetics, if you can. I'm wondering how much they differ, for seal preservatives/additives.

Glasgow 911SC · Glasgow 911SC's Garage

FWIW, here's part of an article that appared recently in the Porsche Club GB's magazine;

"Engine - If seals and piston rings are in good condition i.e. no bad oil leaks and not burning oil - Mobil 1/Castrol RS or any other synthetic OW-40 oil/Halfords 5W-40 fully synthetic. If engine is a bit drippy or uses oil - Halfords fully synthetic 5W-40/ any semi-synthetic 10W-40 or 10W-30/same grades of mineral oil/I 5W-40 mineral oil. NB. These lists are in descending order of preference.
(The Halfords fully synthetic oil is a cheap option, only £16 for 5 litres vs. £30 for 4 litres for the Mobil/Castrol synthetics).
Best advice for a leaky engine - change the seals and gaskets for new ones and use a synthetic oil (one RTV both sides of new gaskets and oil the face of the cover to allow the joint to be broken and re-sealed). Do any mechanical work, like rings and valve stem oil seals and then use a synthetic. Why? It stops wear in its tracks! No mineral oil can do this

Synthetics are good for 25,000 miles or one year so can be cost effective for high mileage users; for lower mileages the Halfords option is a good one.
Benefits are lower fuel consumption & better cold cranking due to lower cold viscosity i.e. OW or 5W spec. This is more than compensated for by a higher film strength to improve engine protection during cold starting... far better than a mineral oil of a thicker cold viscosity! I.e. 15W or 20W, which cause a lot more mechanical drag between moving surfaces. (One slight downside...can give rise to more mechanical noise when engine is cold, but this is mainly on engines with hydraulic valve adjusters, no need to worry though, it is not doing any damage due to higher film strength). Also high temperature protection would appear to be the same as a mineral oil, with the same hot viscosity spec. I.e. -40, but film strength is still much higher than a mineral oil at engine running temperature, giving far superior protection."